Bullying is when someone keeps doing or saying things to have power over another person.
Really, bullying is wrong behaviour which makes the person being bullied feel afraid or uncomfortable. There are a lot of reasons why some people bully. They may see it as a way of being popular, or making themselves look tough and in charge. Some bullies do it to get attention or things, or to make other people afraid of them. Others might be jealous of the person they are bullying.