Metonymy is a scary word for a not-so-scary concept. It's just a type of metaphor in which an object is used to describe something that's closely related to it. So, for example, when you're talking about the power of a king, you might say "the crown," instead. The crown is the physical object that is usually associated with royalty and power.
In fact, once you get used to the concept, you'll spot metonymy just about everywhere:
The key here is that metonymy is an example of figurative language. Shmoop certainly wouldn't want to bring a pen to a swordfight. But we are more than willing to fight violence with words. That's what the Good Guys do, right?
For a more poetic example, check out metonymy in "The Seafarer."