What you say when either
a) You're too tired to say what's happening.
b) You're too sad to talk about what's happening.
c) A lot of stuff is happening, but you don't know the person well enough to tell them.
d) Nothing is literally happening at all.
a) 2:42 AM: What's up?
2:43 AM: Nothing... I'm going to sleep bye.
b) Jane: What's up Sarah?
Sarah (whose boyfriend broke up with her) : Nothing
c) Brianna: Hi, what's your name?
Marissa: Marissa, what's yours?
Blah blah blah, later that day on Facebook or something.
Brianna: What's up?
d) Ed: What's up?
Ed: No, seriously. What's up?
Al: Literally, nothing.