That's What's Up Meaning

(idiomatic, slang) Used to express acquiescence or concurrence.

Example:   "We're gonna have fun tonite right?" / "Yeah. That's what's up!"
2009, Hip Hop Underground: The Integrity and Ethics of Racial Identification, by Anthony Kwame Harrison, page 112
  I choose this particular terminology because of the frequency with which I heard it used either in describing a person (“she knows what's up”) or as a statement of approval in response to a particular thought or idea that was introduced (“that's what's up” )