there are 4 levels of mind called jagrath, swapna, turiya, turiyatheetha, also can be called in modern psychology, conscious, sub-conscious, super-conscious/collective consciousness, now mantra is not just some word, there levels of sound as well, madhyama, vaikhari, pasyanthi, para pasyanti, now gayatri the one we recite can take us to the turiya state of mind, and gayatri mantra as we know has padams or words or stanzas i could not find an alternative for padam in english sorry
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now these padams are just a raw form if we go higher and higher and travel through our mind using this mantra we reach the crude and refined form called as bija akshar, bija is like a small letter or a combination of those eg: hreem, now this crude form of gayatri is what in Sri Vidya called as Panchadasi, this is available in Vari Vasya Rahasyam Text you can refer there.
Now why this is called supreme vidya, you can take any great soul of India, even the devas including Krishna the great avatar himself, they all have obtained this Sri Vidya, because this is the secret of this whole Universe, even Gandhi he himself got it from BotBhole Maharaj, and this Sri Vidya's starting can be from Gayatri, that is the reason why it is the most important mantra and called as veda mata.
if you need more information I can recommend you some books on mantra sastra I am not a good writer probably
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