Om or Aum is considered the most sacred Hindu mantra, appearing at the beginning and end of many Sanskrit prayers and texts.
Om/Aum encompasses everything: the universe, all of creation, all words, and all sounds.
M (mmm): the state of deep sleep.
The symbol Om or Aum has a rich symbolic meaning in itself.
The AUM symbol has three curves (1, 2, and 3), a semicircle (curve 4), and a dot.
Curve 1 is the waking state when you're conscious and aware of everything outside of your body. It's the largest curve because it's the state you're in the most.
The upper curve, number 2, symbolizes deep sleep (sushupti) or the unconscious state. During sushupti, you want for nothing.
Curve 3, the middle curve, sits between curves one (waking) and two (deep sleep) and denotes the dream state (swapna). This is when your consciousness is focused inwards and you're given a different view of the universe inside of you.
The three curves don't just indicate your three states of consciousness. Indian mystic culture believes your manifested reality comes from these three states, so the three states make up your complete physical being.
The dot is a symbol for the fourth state of consciousness, turiya. This state is when you've reached a point of peace and bliss.
This particular state highlights the other three states of consciousness for an "absolute" state, and it takes considerable work and effort to get to this point.
Finally, the semi-circle is a symbol for maya (illusion) and keeps the dot separate from the three curves. It's positioned this way because it's often the idea of maya that keeps you from reaching turiya (or the dot).
The semi-circle is left open at the top for a reason: to show that reaching the highest state of being isn't inhibited by maya.