“What’s up?” is a common informal greeting in every English. There are 4 ways that this can be used.
Because of its informal casual nature, this should not be used in formal settings like work or over the phone.
1. A casual greeting.
This is similar to saying “hi” or “hello.” Although this is more common where interaction is limited – like an online video or a casual presentation.
2. A casual way to ask what is new or happening in your life.
This can also be considered a type of greeting where an interaction is expected – when talking to another person.
A similar phrase to this is: “What’s happening?”
3. A casual way to ask about the purpose of contact/communication.
To me, this is more casual and polite compared to asking “Why are you calling?” or “Why are you here?” Although these are more formal, they can come across as direct and rude.
4. To ask someone what is worrying them.
Interestingly, it can be an expression of concern if you use the appropriate tone.
Similar phrases are: “What’s the matter?” and “What’s wrong?”