Om or Aum
Is a Pranava. A sacred sound that is Brahman (God)
A relates to Brahma, waking state, the past and creation (lots of other things too)
U relates to Vishnu, dreaming state, present etc
M relates to Shiva, deep sleep, the future and destruction and removal of ignorance.
The 3 aspects together make God.
If you read the Mandukya Upanishad it goes in to detail about the holiness of Aum.
The vibration of creation, that is constant, unchanging and eternal
The symbol itself can be broken down in to the holy trinity with the 3 main curves and the little dot (bindu) represents Turiya or infinity