OM — The Primordial Sound
OM (AUM) is the most sacred sound in the Hindu and Tantric traditions — the primordial vibration from which all manifest reality arises, contains, and into which it returns.
OM (or AUM — representing the three states of consciousness: A = waking, U = dreaming, M = deep sleep, and the silence after M = Turiya/pure consciousness) is the Pranava — the primordial sound, the vibrational signature of Brahman/Shiva. The Mandukya Upanishad devotes its entire 12 verses to the meaning of OM. In Tantra, OM is the highest bija mantra — the sound that contains all other sounds, the vibration from which all manifest reality derives. It is used at the beginning of all mantras (OM NAMAH SHIVAYA, OM MANI PADME HUM) as the vibrational invocation of the absolute.
All this universe is AUM. The past, present, and future — all that was, is, and shall be — is AUM.
The Mandukya Upanishad devotes its twelve verses to OM, mapping A, U, and M onto the waking, dream, and deep-sleep states and the silence after M onto Turya, the fourth.
Mandukya Upanishad