To behave means to act in a certain way or to follow the rules of a situation.
When we talk about behaving, it's usually because we want someone to do something that's expected of them. For example, if you go to school, your teacher might tell you to behave during class time, which means not talking too much and paying attention to what they're saying. But if you're at a party with friends, behaving might mean being respectful of other people's property and not doing anything that could get everyone in trouble. In both cases, the idea is that you need to adapt your actions to fit the situation and the people around you.
To conduct (oneself) well, or in a given way.
Example: You need to behave yourself, young lady.
