Misuse means not using something the way it is meant to be used. The use deviates from the rules as they apply in society or in the law. If the abuse is bad, the offender can be punished for it. Abuse can be of objects or people, but also of an office or other things. Here are the most important examples:
In drug abuse, someone uses a medicine or other chemical substance in a different way than intended. Morphine, for example, is needed by doctors for patients in severe pain. If you take too much of it, you get into a state of intoxication. The substance then has a different effect than what was intended.
In the case of abuse of authority, for example, a policeman fines a person because he is his colleague. Some officials, even high-ranking politicians, abuse their office for their own purposes. For example, they allow diamonds to be sought and sold without authorisation. In return, the official demands his own share. This is called corruption.
Objects can also be misused. For example, young people build a small car out of a handcart and use the engine of the lawnmower. This may well work, but the brakes may not be enough. Such misuse can become dangerous or even lead to a fine.