Here is a joke I found from Reddit :

An old Jew who survived the German concentration camps is living in America. Every Monday he goes to his local convenience store and buys a lotto ticket. One day he hits a winner and wins big. The first thing he buys is a life size statue of Adolph Hitler to put in his living room. His family and friends are shocked. They can't understand his decision and think that he's losing his marbles. Finally his grandson asks him "Grandpa, why did you buy a statute of that horrible man and put it in your living room?" The old Jew gets down on one knee, rolls up his sleeve and replies "well son, old Adolph gave me the winning numbers."

Does "losing his marbles" means losing his minds or sort of?

Also, could someone clarify how is this humor?

Thank you again.