There are two contexts when this is used. Firstly as a greeting to someone - this is very casual and would only be used between friends. It's a way of saying " hello, what has been happening in your life since I saw you last". You can reply with anything appropriate - I have been really busy, what about you? Nothing much, I have been taking it easy. Alternatively, the question can be used when the speaker has a reason to think there is a problem, but they don't know what it is yet. Someone at work says to a colleague, you had better go and see how Aninha is doing on that project, I think she is having problems. So the colleague drops by and says 'what's up', meaning what is the problem. Or you phone a friend and you sound upset - the friend will ask, what's up? It is also used when someone thinks there is a problem indicated by another person's behaviour. A colleague gets angry and storms out of the meeting - someone might ask, 'what's up with him?', i.e. what is the problem that made him behave that way.

Hey Aninha, What's up? ''What's up'' is considered to be an expression that means ''What's going on?''/ ''Hello. What's happening?'' / ''What's new?''. / ''What's the matter (with someone)''. This idiom is often used as a greeting that does not require an answer. Here are some examples that may help you: ''So what's up? How have you been?'' ''What's up? How are you doing?'' ''What’s up with him? He looks furious.'' Hope that helps!!!

I think it is equal to the sentence " how are you ? "