Debate is about making good policy, and you can't have a good policy unless you know what the key words of the policy mean. Some words are very difficult to define, and there are huge debates about them. It's easy to understand this concept by thinking about a conversation you might have with your parents. Topicality deals with arguments about what words mean.
Every year there is a different resolution for high school policy debate.
For example, the affirmative might argue that every hungry child in America should be fed. The plan is fine, but it doesn't support the resolution. For example, your history teacher asked you to write a paper about the Civil War. You, however, decided to write a paper about the Vietnam War. But, it isn't usually that simple.