Swami Vivekananda- a key figure in the introduction of yoga to the West- describes The Upaniṣads as 'the Bible of India'. Try to look past the negative connotation that you may carry with the word 'Bible' there, and see him as describing them as highly important texts in Indian culture. There are said to be 108 (a sacred number) Upaniṣads, with the short Kena Upaniṣad, one of the primary texts.
Parts III-IV of the text offer a lovely tale of the Hindu (which is a label that came later) gods and lessons to be learnt from them. The translation offered here- in bold for ease of reading- is by Swāmī Gambhīrānanda:
1. brahma ha devebhyo vijagye tasya ha brahmano vijaye deva amahiyanta
It was Brahman, indeed, that achieved victory for the sake of the gods. In that victory which was in fact Brahman's, the gods became elated.
2. ta aikshantasmakam evayam vijayo ‘smakam evayam mahim eti. tad dhaisham vijajau
tebhyo ha pradurbabhuva tan na vyajanata kim idam yaksham iti.
They thought, 'Ours, indeed, is this victory, ours, indeed, is this glory,' Brahman knew this pretension of theirs. To them It did appear. They could not make out about that thing, as to what this Yakṣa (venerable Being) might be.
3. te ‘gnim abruvañ jataveda etad vijanihi kim idam yaksham iti tatheti
They said to Fire (Agni), ‘O Jātavedā, find out thoroughly about this thing as to what this Yakṣa is ’. He said, ‘So be it’.
4. tad abhyadravat tam abhyavadat ko ‘sity agnir va aham asmity abravij jataveda va
To It he went. To him It said, 'Who are you?' He said, 'I am known as Fire (Agni), or I am Jātavedā,'
5. tasmins tvayi kim viryam ity apidam sarvam daheyam yad idam prithvyam iti
(It said), 'What power is there in you, such as you are?' (Fire said), 'I can burn up all this that is on the earth.'
6. tasmai trinam nidadhav etad daheti. tad upapreyaya sarva-javena tan na shashaka
dagdhum sa tata eva nivavrite naitad ashakam vijñatum yad etad yaksham iti.
(Yakṣa) placed a straw for him saying, 'Burn this.' (Fire) approached the straw with the power born of full enthusiasm. He could not consume it. He returned from the Yakṣa (to tell the gods), 'I could not ascertain It fully as to what this Yakṣa is.'
7. atha vayum abruvan vayavetad vijanihi kim etad yaksham iti tatheti
Then (the gods) said to Air (Vayu), 'O Air, find out thoroughly about this thing as to what this Yakṣa is.' (Air said), 'So be it.'
8. tad abhyadravat tam abhyavadat ko ‘siti vayur va aham asmity abravin matarishva va
To It he went. To him It said, 'Who are you?' He said, 'I am known as Air (Vayu), or I am Mātariśvā.'
9. tasmins tvayi kim viryam iti apidam sarvam adadiya yad idam prithivyam iti
(It said), 'What power is there in you, such as you are?' (Air said), 'I can blow away all this that is on the earth.'
10. tasmai trinam nidadhav etad adatsveti tad upapreyaya sarva-javena tan na
shashakadatum sa tata eva nivavrite naitad ashakam vijñatum yad etad yaksham iti
(Yakṣa) placed a straw for him saying, 'Take it up.' (Air) approached the straw with all the strength born of enthusiasm. He could not take it up. He returned from that Yakṣa (to tell the gods), 'I could not ascertain It fully as to what this Yakṣa is.'
11. athendram abruvan maghavann etad vijanihi kim etad yaksham iti tatheti tad
abhyadravat tasmat tirodadhe
Then (the gods) said to Indra, 'O Maghavā, find out thoroughly about this thing, as to what this Yakṣa is.' (He said), 'So be it.' He (Indra) approached It (Yakṣa). From him (Yakṣa) vanished away.
12. sa tasminn evakashe striyam ajagama bahu-shobhamanam umam haimavatim tam
hovacha kim etad yaksham iti
In that very space he approached her, the superbly charming woman, viz Umā Haimavatī. To Her (He said), 'What is this Yakṣa?'
1. sa brahmeti hovacha brahmano va etad vijaye mahiyadhvam iti tato haiva
‘It was Brahman’, said She. ‘In Brahman's victory, indeed, you became elated thus.' From that (utterance) alone, to be sure, did Indra learn that It was Brahman.
2. tasmad va ete deva atitaram ivanyan devan yad agnir vayur indras te hy enan
nedishtham pasparsha te hy enat prathamo vidamchakara brahmeti
Therefore, indeed, these gods, Fire, Air, and Indra, did excel other gods, for they indeed touched It most proximately, and they knew It first as Brahman.
3. tasmad va indro ‘titaram ivanyan devan sa hy enan nedishtam pasparsha sa hy enat
prathamo vidamchakara brahmeti
Therefore did Indra excel the other deities. For he touched It most proximately, inasmuch as he knew It first as Brahman.
4. tasyaisha adesho yad etad vidyuto vyadyutad a itin nyamimishad a ity adhidaivatam
This is Its instruction (about meditation) through analogy. It is like that which is (known as) the flash of lightning, and It also as though the eye winked. These are (illustrations) in a divine context.
5. athadhyatmam yad etad gacchativa cha mano ‘nena chaitad upasmaraty abhikshnam
Then is the instruction through analogy in the context of the (individual) self: This known fact, that the mind seems to go to It (Brahman), and the fact that It (Brahman) is repeatedly remembered through the mind; as also the thought, (that the mind has with regard to Brahman).
6. tad dha tadvanam nama tadvanam ity upasitavyam sa ya etad evam vedabhi hainam
sarvani bhutani samvanchanti
The Brahman is well known as the adorable to all creatures: (hence) It is to be meditated on with the help of the name tadvana. All creatures surely pray to anyone who meditates on It in this way.
7. upanishadam bho bruhity ukta ta upanishad-brahmim vava ta upanishadam abrumeti
‘(Disciple:) 'Sir, speak of the secret knowledge.' (Teacher:) 'I have told you of the secret knowledge; I have imparted to you that very secret knowledge of Brahman.'
8. tasyai tapo damah karmeti pratishtha vedah sarvangani satyam ayatanam
Concentration, cessation from sense-objects, rites, etc., are its legs; the Vedas are all its limbs; truth is its abode.
9. yo va etam evam vedapahatya papmanam anante svarge loke jyeye pratitishthati
Anyone who knows this thus, he, having dispelled sin, remains firmly seated in the boundless, blissful, and highest Brahman. He remains firmly seated (there).
To firstly put the text into some context, it comes at a time when the primary worship was of gods as deities, such as Agni, Vayu and Indra. Gods which are outside of ourselves, or dualistic. This is before the non-dual philosophies such as sāṃkhya and the tantras, so when one has understanding of those the text may seem somewhat obvious- but no less beautiful. It does mean that it was a text which was rather revolutionary at the time.
We have to understand that what is trying to be put into words is actually indescribable and beyond words (yet paradoxically it is those words too). So what occurs is we see many words for the same thing- some teachings will say Brahman, whilst others Śiva. On an individual level you may see it called ātman, or spoken of as consciousness or awareness. If you want to be possibly triggered, they are all meaning God.
The most important part here in my eyes is the deities realising the actions they had thought were theirs, were actually Brahman. This is also a teaching found later in texts such as The Bhagavad Gītā; that we believe we are our actions when in fact there is something beneath our actions- Brahman. In this text, we can be seen as these deities. We celebrate our victories and morn our losses like they are our own, when they are not ours. They certainly may feel like our own with the understanding that we may have, so this 'new' way of thinking is a means to negate the attachment to those highs and lows.
Let's put it another way. Who creates the deities- or gods? We do. We as 'humans' create a god or numerous gods outside of ourselves. They are OUR creation, so WE are the creator. And that creator here is called Brahman. This can be a daunting thought, and one which may not be realised this lifetime. But there are ways to reach this understanding, as did the deities in the tale.
Notice the behaviour of the deities. I have not seen this observation in any of the commentaries (and that is not saying they don't exist), perhaps as it's somewhat subjective (but then all is really). Nonetheless, I think it carries important message. They began quite bold and thinking 'I am this', 'I am that', but by the end there is an undertone of humility. They didn't clutch on to the idea that they were all powerful, and understood that they didn't understand. Do we have this humility within us, or do we hold onto ideas/thoughts/notions dearly? There is an open and humble quality that they carry here, which seems important in gaining any understanding. Whether that is trying to gain the nature of being, or learning something like how to play the guitar. There are things we are sure we know, and there are things we know we don't know. If I pick up an instrument I've never played, I'm pretty sure I don't know how to play it. The nature of being is a little more complex- we've been 'doing' it- or being it- all our lives, so we can seem sure that we 'know it'. But can we truly know anything?
So the deities here ran experiments essentially. Trying to blow the straw for example. Remember this is a tale, so you trying to blow a straw off a table probably isn't going to guide you to discovering the truth of your existence. But it is this nature of experimenting- or really, playing- which can help us along the way. A willingness to try things out, with that openness and humility, not attached to the notions we currently carry. Observing the results of the experiment and maybe readjusting our thinking accordingly, which might lead to a new experiment. Anyone with children may recognise this. This is how children learn the world around them, and then many of us seem to reach an age where we no longer do this. That's why I call it play- it's as fun as it was as a child! Perhaps we think we know all there is about the nature of things. Perhaps for example, the science taught at school quells our appetite so we no longer feel a need to experiment. Yet in this we lose that childlike joy at the world.
Things start to sound a little more complex in part IV but don't be put off. It talks of the mind, as we need this in order to come to realisations. But part of that realisation is that the mind too is Brahman. So Brahman is realising Brahman. Put it another way, it is a 'state' of becoming aware of awareness. Each time you notice yourself thinking- that is awareness. What the text is describing essentially is turning that awareness back on itself- 'looking' inwards and not outwards to 'sense-objects'.
It potentially gets a little misleading in a lot of the texts, with for example here, the 'flash of lightning'. Even the word 'enlightenment' can imply a sudden beam of light or similar. For some this may be the case. For many though it is much more subtle. Because it is something we always are, it is easily missed. When it's experienced or realised, it can almost feel a laughable disappointment. Longer term it isn't of course, but due to the expectations created (by ourselves and our interpretations of the words written or spoken) in the moment of realisation it may feel like that. It may just simply feel like an 'aaaaaaaaaah'. It will be different for all depending on all that has led before that.
It mentions concentration, but I would suggest that word is a little misleading in the western connotation of concentration. It can imply effort, but is in fact the opposite of this. It's almost a case of removing all effort and simply noticing, without adding thoughts into the equation. Simply noticing, or witnessing that awareness when and as we can. It may say concentration to imply, noticing the lack of noticing as much as possible. Or trying to be aware of when we're not aware. The times we are on auto-pilot; the times we are engrossed in the objects outside of ourselves; the times we are forgetting Brahman if you will. The times we forget we are the creator.
If you want to experience this then maybe try one of the yoga classes I've been putting on YouTube recently. Try to remove any idea of what a yoga class is. These are not keep fit, not stretching, not just relaxation or any of the other things yoga seems to have become over here. They are classes to potentially guide you to the realisations mentioned above. They are different ways to the same revealing as we are all different in what we may need- so if one doesn't strike a chord, don't give up. Also if you have already realised your true nature, they may be a way to a different understanding; each time the experience may be completely different.