What are hump staterooms?
A stateroom on the "hump" refers to staterooms on the part of the ship that extends out from the side.
It's a colloquial phrase for staterooms that are on the part of the ship that if you were to put on its side, those staterooms would look like a hump.
The reason there is a "hump" in the first place is to accommodate a wider area inside the ship for the elevators.
Hump staterooms balconies that follow this hump/bulge shape. The balconies and rooms aren't any larger, but those on the leading or trailing arc of the hump will have a view down the side of the ship and some find that more desirable.
On some classes of Royal Caribbean ships, the balcony space of these statrooms will have more space than usual.