What is the meaning of the Om symbol ?

* OM is the single most ubiquitous symbol in all of Hinduism

The symbol Om, Ohm or Aum is believed to be the primordial sound that emanated during the creation of the Universe

* It is made up of three separate sounds, and saying these together makes Om the ultimate mantra.

* Saying the three sounds together in the right way helps to awaken the inner self, the atman, which is a spark from the divine Brahman. When said in this way, Om is called Pranava, the sacred sound (sacred humming).

* It is the representation of Brahman, who is unreachable and unknowable. By using the symbol (or saying the word), Hindus can approach Brahman in both the mystical and earth-bound planes.

* The symbol has enormous significance in Hindu life, appearing everywhere – on temples, on amulets worn by almost everyone and even painted onto the tongue of newborns using honey, to welcome them into life.

=> The Mandukya Upanishad has an excellent discussion on the significance of the Om sound. 

Om ityetadaksharam idam sarvam, tasyopavyakhyanam bhutam bhavat bhavishyaditi sarvam omkara eva.

The Imperishable is OM, and it is 'all this'. Everything else, whatever be of the past, present or future, is like an exposition, explanation or commentary on the meaning of this great Truth – the Imperishable Om. Sarvam Omkara eva: Everything is Om, indeed. This is how the Upanishad begins. Om itiyetadaksharam idam sarvam: All this, whatever is visible, whatever is cognizable, whatever can come within the purview of sense-perception, inference or verbal testimony, whatever can be comprehended under the single term, creation – all this is Om.