"What's up?"

This is a casual expression used in informal conversation, NOT formal situations.

Translation:

How are you?

Cultural Note:

Though Americans ask "What's up?" and "How are you?," it is meant as a greeting to be polite. Standard responses are "Fine," "Great," "Ok," "I could be better."  Unless you are close friends, people do not expect a detailed response about how you REALLY feel.  We are just saying "Hi."