What it means:
In this context, "up" means "happening". "What's up?" is a casual greeting that means
"How are you doing?"
"What are you doing?"
- "What things are happening around you?"
People use this phrase mostly when seeing friends or acquaintances.
How to respond:
Sometimes, people say "What's up" as a generic casual greeting. The typical response is "not much" (even if you are doing something). You can also reply with any casual greeting:
- "What's up"
- "'sup" (short version of "what's up")
Other times, people want to have a conversation, and might expect an answer. In this case, you can say what you are doing/how you are feeling/etc, in a casual manner:
- "I'm just cleaning my apartment"
- "feeling sick, might have caught the flu"