When you say you're 'after something', it means you want it, especially in situations like shopping, or when buying things. Like this:

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A: I'm after a pair of formal leather shoes.

B: Come this way sir. We have a great selection.

A: Hello, what are you after?

B: I'd like a glass of white wine please.

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after在我們所學過的意思中，大多指位置或是時間，但這邊的after=want，這個用法通常用在購物時！

但在美國口語中，＂be(或get)after someone (或something) ＂是指追着某人，纏着某人或妄想得到(别人的东西)。

From BBC learning English