The mantra Aum stands for the manifest cosmos & the unmanifest Absolute
The following is a translation of the Mandukya Upanishad from The Principal Upanishads by S. Radhakrishnan. The introduction reads: “This Upanishad belongs to the Atharva Veda and contains twelve verses. It is an exposition of the principle of Aum as consisting of three elements, a, u, m, which refer to the three states of waking, dream and dreamless sleep. The Supreme Self is manifested in the universe in its gross, subtle and causal aspects. Answering to the four states of consciousness—wakefulness, dream, dreamless sleep, transcendental consciousness—are aspects of the Godhead, the last alone being, all-inclusive and ultimately real. The Absolute of mystic consciousness is the reality of the God of religion. This Upanishad by itself, it is said, is enough to lead one to liberation.”
1 Aum, this syllable is all this. An explanation of that (is the following). All that is the past, the present and the future, all this is only the syllable Aum. And whatever else there is beyond the threefold time, that too is only the syllable Aum.
2 All this is, verily, Brahman. This self is Brahman. This same self has four quarters.
3 The first quarter is Vaishvanara, whose sphere (of activity) is the waking state, who cognizes external objects, who has seven limbs and nineteen mouths and who experiences gross (material) objects.
4 The second quarter is taijasa, whose sphere (of activity) is the dream state, who cognizes internal objects, who has seven limbs and nineteen mouths, and who experiences the subtle objects.
5 Where one, being fast asleep, does not desire any desire whatsoever and does not see any dream whatsoever, that is deep sleep. The third quarter is prajna, whose sphere is the state of deep sleep, who has become one, who is verily a mass of cognition, who is full of bliss and who experiences bliss, whose face is thought.
6 This is the lord of all, this is the knower of all, this is the inner controller, this is the source of all, this is the beginning and the end of beings.
7 (Turiya is) not that which cognizes the internal (objects), not that which cognizes the external (objects), not what cognizes both of them, not a mass of cognition, not cognitive, not non-cognitive. (It is) unseen, incapable of being spoken of, ungraspable, without any distinctive marks, unthinkable, unnameable, the essence of the knowledge of the one Self that into which the world is resolved, the peaceful, the benign, the non-dual, such, they think, is the fourth quarter. He is the Self; He is to be known.
8 This is the Self, which is of the nature of the syllable Aum in regard to its elements. The quarters are the elements, the elements are the quarters, namely the letter a, the letter u and the letter m.
9 Vaishvanara, whose sphere is the waking state, is the letter a, the first element, either from the root ap, to obtain, or from being the first. He who knows this obtains, verily, all desires; also, he becomes first.
10 Taijasa, whose sphere is the dream state, is the letter u, the second element, from exaltation or intermediateness. He who knows this exalts, verily, the continuity of knowledge, and he becomes equal; in his family is born no one who does not know Brahman.
11 Prajna, whose sphere is the state of deep sleep, is the letter m, the third element, either from the root mi, to measure, or because of merging. He who knows this measures (knows) all this and merges also (all this in himself).
12 The fourth is that which has no elements, which cannot be spoken of, into which the world is resolved, benign, non-dual. Thus the syllable Aum is the very Self. He who knows it thus enters the Self with his self.