Reasonable describes someone or something that's sensible and fair, like your teacher who gives reasonable homework assignments — they don't take you forever to do and they relate to what you are studying.
If you're reasonable, you have good sense and judgment. A reasonable decision is rational and thought out, like your mom's reasonable rule about not eating crumbly foods in her car. When you describe a store's prices as reasonable, you mean they're fair — not too high. And if you are given a reasonable amount of time to do a project for school, you have no excuse for it being late.