AUM represents manifest and un-manifest aspects of Brahman.+

Sage Goudpada:~ The non-dual Atman is realized when the individual Self  (Jiva) is awakened from its ignorance. Atman is unborn, dreamless, sleepless, and motionless, and is beyond duality. It is cognition at its purest. It is Brahman- Ayam Atma Brahma, this Atma is Brahma; Thus epitomizing the core of Upanishad teachings.
 

Sage Sri, Gaudapada expands further on these states of consciousness. The Self is AUM. It represents the manifest and unmanifest aspects of Brahman. It is the single syllable that symbolizes and embodies Brahman, the Absolute Reality. It is the Pranava that pervades all existence and is our very life breath.

Vaisvanara in waking the state is A the first part of AUM, One, who realizes this, attains his desires.

Teijasa in the dream state is U the second part of AUM. One, who realizes this, attains knowledge.

Prajna in deep sleep is M the third part of AUM, concluding the sounds of the earlier two parts. One, who realizes this, attains a compressive understanding of all.

The Syllable AUM in its entirety stands for the fourth state, Turiya the one beyond the phenomenal existence, supremely blissful and non-dual.

AUM in its integral whole stands for the fourth state which is transcendental, devoid of phenomenal existence, and is the source of all existence. AUM represents Ultimate Reality. AUM is thus verily the Self itself. One who realizes this merges into that Self. Meditate on AUM as the Self.

Ignorance is the cause of duality.   The duality is the cause of the illusion. The illusion is the cause of experiencing birth, life, death, and the world as a reality.

Self-knowledge leads to Self-awareness.  The quest for truth is not the quest for acquiring the dualistic truth within the world.    The seeker must have an intense urge to realize the truth which is beyond form, time, and space.

 The dualistic knowledge hides the truth beyond form, time, and space.  The practical life within the practical world is nothing to do with the ultimate truth or Brahman.   

The dualistic knowledge within the dualistic illusion is not Self-knowledge or Brahma Gnana or Atma Gnana.

This vast cosmos with all its contents (atom, molecule, sun, moon, planet, and star) all are created out of single stuff. That single stuff is the Soul, which is present in the form of consciousness. Knowledge of the single stuff is Self-knowledge or Brahma Gnana or Atma Gnana.   

Consciousness is ever-present. Without consciousness, the world, in which you exist ceases to exist.   Consciousness is Self-evident. It is not established by extraneous proofs. It is not possible to deny consciousness, because it is the very essence of the one who denies it. Consciousness is the basis of all kinds of knowledge, presuppositions, and proofs. Consciousness is everything. Thus, consciousness is the ultimate truth or Brahman or God in truth.:~Santthosh Kumaar