"What's up" is slang and is used very commonly by younger generations and amongst friends. It means "what is happening now/in your life?", "What are you doing?".
Don't take it literally because if we analyze it, it will confuse you!
This is why:
What + is + up
What = pronoun/determiner: asking for information
is= conjugated verb "to be"
up= adverb/prepostion/adjective --> opposite of down, a higher place.
Next time someone asks you "what's up?" you can respond by saying, "nothing much" or "a lot!" depending on you of course :)
Did that help?