This is the contraction for have, and is seen in the contractions I've, you've, we've, they've, who've where it's pronounced [v]. It's also commonly seen in the forms could've, should've, would've, must've, where it's pronounced [ǝv].

____________Note that it's only rarely written in the negative forms: couldn't've, shouldn't've, wouldn't've, can't've, unless the author is citing someone's exact speech in a dialog, for example, or when they're used informally in texts or chats.

It's important to bear in mind that the verb have has two distinct usages:

The contraction 've is used for the latter — to form the present perfect, and not to express possession. You may hear people say something like You've a beautiful smile in old movies and other dialects of English, but in modern American English, the only time we use 've to express possession* is in the expression You've no idea

Incidentally, this is the only time we use the inverted form have you to question possession: Have you any idea how much this cost me? In every other case, we use the full forms you have and do you have

____________*Note that to have got also expresses possession, but here got is considered a participle and is the part that expresses possession, not have.