"What does it really mean when one asks - what’s up?" What should be my reply? What I am busy with or upto? I am assuming its informal usage. Also I doubt if one should use it in formal meetings. Any guidance/suggestions ?
"What's up" when used as a greeting follows the pattern of How do you do. It may appear at first glance appears to be a question, but it is in fact stated and given as an idiomatic set phrase, the correct response to which is to repeat the question:
John: Hey Matt! What's up?
Matt: Yo John! What's up? How've you been? I've not seen you since Monday!
"What's up" is an informal greeting, and should only be used in informal situations. If in doubt, do not use it to greet people that have not greeted you with "What's up" in the past. For more formal situations, use hello or hi.