Desu- polite “is”
It means “stupid” in Japanese. The sentence has no subject, so I’m not sure what is stupid. If the sentence is “watashi wa baka desu.”, It means “I am stupid”. If it is “anata wa baka desu.”, It means “You are stupid”.
it normally would be translated to “you/he/she/it is stupid/foolish” in japanese but it all depends on the context.