The verb to ban means to forbid something from being or happening. The word can also be used as a noun. A school board might put a ban on all books that refer to Jefferson's mistress if they don't want anyone reading about her.
If your parents are upset by how much time you spend online, they might ban computers in the house. But if you're smart, you can just use your phone or laptop at school. To ban something is to put a ban on it. Something with a ban on it is said to be banned. A ban can vary from your grandmother's ban on people wearing shoes in her house to a government's ban on printing certain stories in the newspaper.