Rhyming slang is a form of slang used a lot in London and the rest of the UK. It is constructed by replacing a common word with a phrase of two or more words, the last of which rhymes with the original word. Afterwards, the second word is dropped, leaving just one word which seems to have nothing to do with the original. 

For example: the original word feet rhymes with plates of meat. Dropping the 'of meat' part leaves you with just plates as your slang word.

Another example that I didn't know is the phrase "blowing a raspberry", which comes from raspberry tart, which rhymes with fart.