Used when you like something, or are 'down' to participate in a certain activity. The phrase can also be used as a filler when no logical explanation can be attained.
Bennett: Dude, are you down for some Taco Bell?
Bryce: I'm 'bout it.
Officer: Sir, you realize you are in possession of twenty-two types of illicit drugs?
Guy:...I'm 'bout it.
Me: IM SO 'BOUT SOME PIZZA RIGHT NOW