What does usted es un idiota mean in English?

It should actually be "usted es una idiota" but it means "you are an idiot." usted is the formal way to say "you." if you want to be informal with a person you've known for a while such as a friend or someone younger than you, you would say "tú eres una idiota" but only idiota if you're talking to a girl. It would be "Tú eres un idioto" if it were a boy you were speaking to.