What is AUM or OM?

ओमितयेतदकशरमिदँ् सर्वं तसयोपव्याख्यानं भूतं भवद्भविष्यदिति सर्वमोङ्कार एव ।

यच्चन्यत् त्रिकालातीतं तदप्योङ्कार एव ॥ १ ॥

"The word OM is the Universe. Everything that exists in the past, present and future is OM. And that which exists beyond the 3 fold division of time is OM."

- Mandukyopanishad, 1

AUM or OM is the sum of all possibilities and all probabilities. Om is the mother of all names, all forms, and all ideas; of time and space; of everything that ever was, is, will be, or could be. Om is the mother of Dharma, the principle which creates, sustains and dissolves all of creation, over and over, again and again.

AUM is the structure of the universe.

The universe exists ultimately as the relationship between subject and object. The subject is pure consciousness or Purusha. Purusha is represented by the sound "A" in AUM. The object is Prakriti or the manifest universe. In AUM, the sound "A" is shaped by the sound "M", just as pure consciousness or Purusha is shaped by Prakriti. The relationship between the two is called Dharma, the principle of evolution which sustains the universe, represented by "U" in AUM.