One of the people who works under you came to your office to ask a question. He asked you "Can I ask you something?" You were typing an email, but it wasn't that important, so you answer him like this.
Sure. What's up?
"Sure" means "yes". But it's more casual. Just answering "yes" to a question can sound as if you are angry.
"Sure" also has some of the meaning of "of course" - meaning that it's obvious and expected for the answer to be "yes". But "of course" is a stronger response than "sure".
The phrase "What's up?" has many different uses, but one of them is to respond when someone asks to talk with you:
A: Hey, can I talk with you for a minute?
B: Yeah, what's up?
"What's up?" sounds light and casual. You can use this with people who you work closely with, who you're friends with, or who work for you.
A somewhat more formal answer to this question is:
Sure, what can I do for you?
Or, if the person you're talking to seems upset, you can ask:
What's going on?
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