"Can't" is a contraction of "cannot," which is used to express an inability or lack of permission to do something. It indicates that a person is not able to perform a specific action or that an action is not allowed. For example, saying "I can't swim" means that the person does not have the ability to swim. In everyday language, "can't" is often used in various contexts, such as in school when a student says, "I can't go outside," meaning they are not allowed to leave the classroom. It can also express personal limitations, like "I can't lift that heavy box," indicating a physical inability.