When I wear a shirt or some other garment with the OM symbol

I invariably have someone ask me what it means.

The symbol is a Sanskrit term and it is said to be the essence of speech. The sound of OM or AUM is said to be the sound of the universe and when sung is composed of three syllagles combined into one.
"Aaaoooummm."

 The symbol consists of three curves, one semicircle, and a dot. The three curves represent the three stages of consciousness; the lower curve is awake, the upper curve is deep sleep, and the tail-like curve represents dreaming sleep.  It is said that these are the three states of consciousness.

The semicircle above the consciousness curves represents maya  (illusion). In other words, the three states of consciousness are merely an illusion. The dot above maya represents God or the Absolute.  So, when you get rid of the illusion (maya) that you are consciousness you become one with God. This union is achieved through the practice of yoga.