Jailbait images are pictures of young people who are not yet adults, but who may look more mature than they really are. The term "jailbait" comes from the idea that these pictures can be used to lure older people into illegal or inappropriate activities, which could lead to the young person being taken advantage of or hurt.
When people take or share jailbait images, they can be breaking the law, because it is illegal to exploit children or young people in this way. So if you ever see these kinds of pictures, it is important to tell a grown-up you trust, like your parents, a teacher, or a police officer, so they can make sure everyone stays safe.