What does vous sont fou fou mean?

- vous êtes fou (formal you, singular) = you are mad / crazy;

- vous êtes fous (you plural) = you (all) are mad;

- Ils sont fous (plural - not possible in the singular) = they're mad;

fou-fou (where you double the Word) is milder than 'fou'. It is a familiar Word employed for for instance to an excited (in an often positive way) child.