Something is relevant if it's appropriate or connected to the matter at hand. Relevant things are helpful and on point.
Relevant things are appropriate and make sense at that particular time. In the middle of history class, your teacher loves to get relevant questions: questions that have to do with the material. The teacher won't be thrilled to get a question about math: that's not relevant. You can also say a song or movie is relevant if it's connected to current events or issues people are talking about. When something is relevant, it just fits what is happening.