Irrelevant means not connected to what matters in a particular situation. When something is irrelevant, it has nothing to do with the topic at hand or the decision being made.

If your class is discussing the causes of the American Revolution and someone starts talking about their favorite video game, that's irrelevant: it doesn't relate to the subject everyone's focused on. When a detective investigates a crime, she has to sort through lots of information to figure out what's relevant evidence and what's irrelevant detail. The fact that the suspect likes strawberry ice cream is probably irrelevant, but the fact that he was seen near the scene matters a great deal.

In an argument or debate, people sometimes bring up irrelevant points to distract from the real issue. If you're discussing whether to extend recess and someone says, “Well, the gym teacher has a red car,” that's completely irrelevant to the question. Learning to spot irrelevant information helps you think more clearly and stay focused on what actually matters.

The opposite is relevant, meaning connected and important to the situation. When you're writing an essay or solving a problem, you want to include only the relevant facts and leave out anything irrelevant that would just clutter up your thinking.