British slang is informal language, not really part of standard English. Not the kind of language which you typically use in more formal situations like business meetings or whatever. If you have spent any length of time living in the UK, if you've got English friends or British friends then you might recognise some of these words and expressions. It's difficult with slang, because you've got to get it right. You've got to use ones that people still use. It's no good using slang from 50 years ago, because you're just going to sound really weird.