Guerrilla theater is like playing pretend but in front of people to make them think about important things. It's like when you and your friends dress up and act out a story, except instead of just having fun, you're trying to make people pay attention to something that's happening in the world.
For example, if there's a problem with pollution in a city, a group of people might dress up like fish or trees and perform a play on a busy street to get people to think more about pollution. They wouldn't tell anyone they were doing it beforehand, so it's a surprise and people will stop to watch.
The goal of guerrilla theater is to make people think and change their minds about things. It's like a secret weapon to help people fight for things that are important to them.