It's used for a five pound note (in the UK) or five dollar note (in the US). You can use it in contexts involving money.

Fiver, like everyone stated is more of a British term. In the US we say five bucks or a Lincoln. You use in it to say how much something cost. That book cost a fiver. You owe me a fiver.

So this is commonly used in the UK for a five pound note. In the US incidentally we don’t say this … Instead we say “five bucks”. You could say something like “Just give me a fiver, and we’ll call it even.”

In the United States, it means "a five-dollar bill." In England, it means "a five-pound note." I would not call them "slang," just "informal." You could say to a friend, "could you lend me a fiver?" Personally, I wouldn't--I'd just say "could you lend me a five?"