If you complete a mission, you'll receive both a star and a reward, and once you complete all three missions you'll even receive an extra reward! Once you complete all three missions in the quest, you'll move on and unlock the next one. When you complete a quest, you may play it again if you want to, but keep in mind that you won't receive the same rewards again. Of course, the more you progress and complete Quests, the harder it will become, but the better the prizes will be!!