Good answer storm, but it doesn't carry the same meaning as 'How are you?' and 'What's new?'. Greetings are almost never meant to be taken literally, but the closest translation would be 'what's happening'. When someone says 'What's up?' the answer is most commonly 'Not much' (Nothing exciting is happening). You will also hear people reply ‘I’m going to…..’ or ‘I’m doing….’. You will also quite likely hear ‘Just chillin’’. Also see this dialogue: ‘Hi.’ ‘Hello.’ ‘How are you?’ ‘Not very good.’ ‘Why what’s up?’ ‘I have a headache.’ In this dialogue it has the meaning of ‘What is wrong?’

I would answer: "Not much. What's up with you?" You can see the parallel question and answer in Q. How are you? A. Fine, thanks. How are you? (or: And you?) Q. What's up? A. Not much. What's up with you? Both "How are you?" and "What's up?" are common greetings. But "What's up?" is informal. It is safe to say "How are you?" to anybody. But I would only say "What's up?" to a friend or at least someone on my own social standing. Neither "How are you?" or "What's up?" expects a genuine answer. These, and other greetings, are (necessary) formalities that we ask and answer when we see someone, usually for the first time each day, or for the first time after not seeing someone for an amount of time. Since neither question expects a genuine answer, their function is identical. And one does not have to pay attention to their literal meaning. * BUT: if you are asking this to a friend or to someone who likes to complain, you might get a genuine answer back! But this is usually against the social norm. Similar greetings & answers include: Q. How's it going? A. Great. (Or: It's going well. Or simply: It's going.) How's it going with you? Q. How is everything? A. Fine. How are things with you? Q. How have you been? A. Great. Or: Fine. And you? Q: What have you been up to? A. Not much. What about you? Q. What's new? A. Nothing much. How about you? Q. What's happening? A. Nothing much. What's happening (or What's up) with you? Q: What's going on? A. Not much. What's going on with you?

Thank you guys for your answers, now I understand it :)

That is what it means. "What's up" is a (don't want to say slang) for "how are you?" And can also be translated as "What's new" ("What is new about you" is the meaning then)