Metonymy is a figure of speech (i.e. a thing we do to make language more fancy or poetic) that replaces the name of something with the name of something else it’s closely associated with, but isn’t a part of. Got it? Nope, me neither. Let’s look at a famous example of metonymy: ‘The pen is mightier than the sword’ (from Edward Bulwer Lytton’s play Richelieu, BTdubz – good to know in case it ever comes up in a pub quiz). The ‘pen’ refers to the written word, and the ‘sword’ is military power – so both ‘pen’ and ‘sword’ are representations of something else that they’re associated with. (Personally I’d rather have a sword than a pen in most situations, for example a zombie apocalypse, but that might just be because I watch too many horror films.)
Some other more modern examples of metonymy are:
‘Hollywood’ to refer to all celebs, film directors and producers and so on
‘top brass’ for management types
‘new blood’ for new people or ideas
‘the big house’ for prison.
It’s probably fairly unlikely that you’ll find yourself in a situation where it might be an issue, but try not to confuse metonymy with synedoche (pronounced si-nek-duh-keen – although I’ve never heard anyone say it out loud). Synecdoche is when you talk about a thing using the name of one of its parts. So it’s a bit more literal than metonymy – like calling business people ‘suits’, or your car your ‘wheels’ (although if you do either of those I’m afraid you might be a prick).