A simpleton is an idiot — a person without much common sense or intelligence.
This is one of many words — such as moron, dummy, and dimwit — that insult a person's intelligence. A simpleton is the opposite of a genius. However, the word simple implies more than lack of intelligence; it suggests being innocent or naive too. So a simpleton could be considered a hillbilly or yokel as well as a dullard or dunce.