(something) is up means something is happening. "What's up?" is just another way of saying "What's happening?" / "What's going on (that might interest me the speaker)?"

 

Sth is up意思是sth is happening（某事）發(fā)生。“What's up?”其實(shí)是“What's happening?（發(fā)生了什么）”，“What's going on(能勾起發(fā)問者興趣的事情)？”的另一種形式。

That is an expression that has about as much meaningfulness as "How are you?" or "How's it going?" 

這是一個(gè)和"How are you?"、"How's it going?" 基本上有相同意義的表達(dá)。

All, including "What's up?" are used as greetings. 

它們所有的，包括"What's up?"都可以用作打招呼。

It has however fallen into common usage both in the US and UK.

它在美國和英國都已成為常用用法。

就問候這一層面來講，你的回復(fù)只是需要簡單的回應(yīng)它的打招呼，或者是說明情況一切正常，沒有什么特別的事情發(fā)生即可。

-What's up? 

忙什么呢？

-Not much / nothing, how about with you?

沒啥，你呢？

還有一些情況就是你身邊確實(shí)發(fā)生了些什么，或者你手頭正在做什么，你就可以直接交代出來，

比方說你正在洗車，然后有人見到你問候你

-What’s up?

忙什么呢？

你就可以說

- Not too much, just trying to get my car clean.

沒什么，就是清理清理我的車。

又或者你正在街邊散步

有人過來跟你打招呼

-What’s up?

忙什么呢？

你就可以說

- Just trying to get a little exercise in. What’s up with you?

就鍛煉一下，你呢？