A metonym is a figure of speech in which an object or idea represents a larger concept that's related to it. Do you know someone who refers to her car as her "ride"? She's using a metonym.
Once you start looking for metonyms, you'll realize how common they are. Ever heard the phrase "the pen is mightier than the sword"? Here, "the pen" is a metonym for writing in general, and "the sword" stands for war or violence. Have you ever read a book that referred to a monarch as "the crown"? Or heard a newscaster talk about the White House making a comment? These are also metonyms. The Greek root, metonymia, means "a change of name."