What does it really mean when fat people are hungry?

The all-you-can-eat buffets are going to make money.

That they are hungry. This is the dumbest question I have ever seen on this site.

The same thing it means when anyone else is hungry. It means they are hungry OR they may be thirsty (many people mistake thirst for hunger). It takes a lot of calories to support a large body, so if you are implying you don't understand why they feel hunger, that doesn't make sense. The heavier you are, the more calories your body needs to maintain that weight, thus you'll feel hunger when your body needs you to eat (just like people of any other weight).

They should eat.

Just like anyone else... it means they're hungry.