"What's good?" is a greeting similar to "what's up?" or "how are you?" When asked "What's good at (name of a place)?" your reply might include what you're doing, how you're doing, anything of note about the place of interest , etc. "Nothin, chillin" is also an acceptable response, though some may look upon you as cold, uninterested, or simply unoriginal.
Ben: Kacey, What's good?
Kacey: Jay-Z is playing on campus and we're high as shit.