Censorship blocks something from being read, heard, or seen. If you've ever heard the sound of bleeping when someone is speaking on television, that's censorship.
To "censor" is to review something and to choose to remove or hide parts of it that are considered unacceptable. Censorship is the name for the process or idea of keeping things like obscene word or graphic images from an audience. There is also such a thing as self-censorship, which is when you refrain from saying certain things — or possibly re-wording them — depending on who is listening.