An idiot, in modern use, is a stupid or foolish person.
It was formerly a technical term in legal and psychiatric contexts for some kinds of profound intellectual disability where the mental age is two years or less, and the person cannot guard themself against common physical dangers. The term was gradually replaced by the term profound mental retardation (which has itself since been replaced by other terms). Along with terms like moron, imbecile, and cretin, its use to describe people with mental disabilities is archaic and considered to be offensive.