"What's up?" Not much. Not too much Nothing much. Not a whole lot.

It means "What are you doing?", but most people just use it nowadays as a way to say "hi" and to start a conversation. The most common answers tend to be "nothing" or "not much".

The answer is "nothing" (most common response, maybe 99% of people say that)

"what's up?" is "what happens?" or "how are you?" if someone asks "what's up?" you respond "I am ok", "nothing".

As others have said, usually this is just a casual greeting that follows the initial hello. The usual response would be "nothing much". However, if you were looking agitated or upset, this greeting would also carry the sense of "What's wrong?".