A reason explains why you do something. The reason you go to school is to learn things (and because it's the law).
Reason usually has to do with thought and logic, as opposed to emotion. If people think you show good reason, or are reasonable, it means you think things through. If people think you have a good reason for doing something, it means you have a motive that makes sense. Reason can also be a verb. You might reason something out, which means that you look at all sides of the issue and then and decide what to do.