A motive is something that causes us to act or behave in order to reach a a goal or desired endpoint. It comes from the latin word that means 'moving'. A motive is the reason WHY you do something.
For example, a motive for exercise is better health and weight loss. In criminology a motive is the reason an individual committed a crime or offense. For instance, the motive for someone who robbed a store is most likely that they needed money.