Hi Shenjja

It means that he has lost his mind. He has lost his brain. He has gone mad. :) 

But a child can also lose his marbles. So the boy has lost his marbles. Marbles are little round glass balls used to play with. 

It would depend on the context. Are you trying to explain a man that has gone mad or a boy that has literally lost his bag of marbles that he plays with. 

It is an idiomatic expression which basically means that someone is a little crazy or mad, or is angry for some reason!. This could be referring to either a temporary or permanent situation.

For example, 'My wife couldn't find her car keys this morning and she lost her marbles'. - temporary anger.

'The car accident damaged his brain and he has lost some of his marbles'. - permanent loss of some mental functions.

I hope this helps!