Aum is the vibrational sound of the cosmic machine of creation and destruction. Aum is the name of God in its unmanifested form. Aum comprises all the manifested and unmanifested worlds.
Aum means nothing and yet it also means everything.
Aum is the most mysterious word in the Universe, thought to be the eternal sound.
Aum has three sounds A...U....M.
A - Signifies the beginning of the universe
U - Signifies the life period of the universe
M - Signifies the destruction of the Universe
Hence AUM is the only word which represents the three main works of God: creation, operation, and destruction of Srishti (the Universe). It represents the three first forms of God: Brahma (creator), Vishnu (operator) and Shiv (destructor).
Most of the mantras in Hinduism start with the word AUM because AUM is the consciousness while the other words in a mantra are Nature or Prakriti. AUM in itself is a complete Mantra and its chanting is sacred and brings enlightenment. It is truly called the Maha Mantra.
It does not belong to any language; it is the primordial vibration of the consciousness which created the universe.
AUM has four steps: A.... U.... M and a final step being silence
A = Physical plane , wake state of mind, Beginning, Brahma
U = Astral Plane, dream state of mind, Manifestation,Vishnu
M = Causal Plane, dream-less state, Destruction, Rudra
The Fourth unspoken sound = Turiya, beyond the universe, Enlightened state
AUM is a Sagun Mantra, Nirgun Mantra and Beeja Mantra, ALL IN ONE.
Recitation of Aum brings peace and happiness and finally Enlightenment.