It could mean th following: 1.(idiomatic, colloquial) What are you doing?; what is happening? 2.(informal) How are you?; how are you feeling? 3.(idiomatic, colloquial) What’s the matter? 4.(rhetorical question, colloquial) A casual greeting with a meaning similar to how are you? or nice to meet you. In my opinion it's slang and I wouldn't use the phrase to address my boss. I might use it, however, to address my mother in law - if I dislike her :)

It's very informal. I would not use it when meeting anyone for the first time. It's used among friends or family members close in age. And "what's up babe?" is disrespectful if you were saying it to a woman who respects herself. Just don't use it. You can use "what's up?" to mean "How are you?" when you talk to close friends.

"What's up?" is informal. How informal would depend on who you ask, but you wouldn't say it to your boss, or your mother in law.

Thank you, very much.

You should write the following: What does this phrase mean? or What is the meaning of this phrase?