what's up是现在较为流行的问候语，相当于how are you，但是how are you 已经过时了，是相当老土的问候方式。what's up差不多是“最近怎样”“近况如何”的意思，也等于what's going on?what's happening？通常没什么事就会回答"Not much"或者"Nothing"。有时问的人并不期待你的回答，所以也可以不回答~~.

It has two meaning. One is--What are you up to? (You can answer:I am fine) Another one is--What's wrong?(you can answer:Nothing)