Aum - Pranava Manthra

The symbol of Om contains of three curves, one semicircle and a dot. The large lower curve symbolizes the waking state; the upper curve denotes deep sleep (or the unconscious) state, and the lower curve (which lies between deep sleep and the waking state) signifies the dream state. These three states of an individual’s consciousness, and therefore the entire physical phenomenon, are represented by the three curves. The dot signifies the Absolute (fourth or Turiya state of consciousness), which illuminates the other three states. The semicircle symbolizes maya and separates the dot from the other three curves. The semicircle is open on the top, which means that the absolute is infinite and is not affected by maya. Maya only affects the manifested phenomenon. In this way the form of Om symbolizes the infinite Brahman and the entire Universe

Uttering the monosyllable Om, the eternal world of Brahman, One who departs leaving the body (at death), he attains the superior goal -Bhagavad Gita, 8.13

The Mantra

Aum
Bhuh Bhuvah Svah
Tat Savitur Varenyam
Bhargo Devasya Dheemahi
Dhiyo Yo nah Prachodayat
[ The Rig Veda (10:16:3)]

The Meaning
"O thou existence Absolute, Creator of the three dimensions,
we contemplate upon thy divine light. May He stimulate
our intellect and bestow upon us true knowledge."

(an extract)