The om (also aum ) has immense importance and is considered the sacred breath in Hinduism and Buddhism. Both religions believe it to be the spritual word or breath that began the process of creation, hence the most important sound in meditation on life and spirituality.
Each sound represents, respectively, Brama The Creator, Vishnu The Preserver, and Shiva The Destroyer, the three principal idols in the religion. In meditation, the om (AUM) sound is created using all three elements of the human voice: the A comes from the throat, and the U and M from the lips.

Other meanings that the om represents include:

Knowledge
Strength (Shakti)
The sun and light
The self and soul
Time (the past, present, and future)
Consciousness (waking, dreaming, conscious ,subconscious ,deep sleep and unconscious)

with mandala om is a symbolic representation of the connection between man, the divine, and the universe.