"WHAT DOES İT MEAN?" <--- these capital letters mean you are yelling. "What up?" is a casual greeting, and a short form of "What's up?" or "What's news?" You can understand it to mean "naber?" or "ne var ne yok?" However it's a typically US greeting, so speakers from other countries (like me) won't use it. Of course, everyone will understand you anyway.

It's a greeting and means "how are you"

it's like "Nasılsın?":-)

Idiom meaning "How are you?"