Om symbolizes the four states of being. It is the Divine sound.
In this video Gerald is explaining the Fourth State - Turiya. The other three states are waking, dreaming and sleep.
A little excerpt from one of my favorite gurus, Ramana Maharshi. "Be as you are, The Teachings of Sri Ramana Maharshi, edited by David Godman":
Q: How are the three states of consciousness inferior in degree of reality to the fourth [turiya]?
A: There is only one state, that of consciousness or awareness or existence. The three states of waking, dream and sleep cannot be real. They simply come and go. The real will always exist. The 'I' or existence that alone persists in all the three states is real. The other three are not real and so it is not possible to say they have such and such a degree of reality. We may roughly put it like this. Existence or consciousness is the only reality. Consciousness plus waking, we call waking. Consciousness plus sleep, we call sleep. Consciousness plus dream, we call dream. Consciousness is the screen on which all the pictures come and go.