The aft is the back, or toward the back, of an airplane or ship.
In many, if not most, commercial aircraft the washrooms for the plebeians in cattle class (aka economy class) are in the aft of the plane. This is important information to have, particularly if you’re on a long flight, because airlines frown on customers peeing in their seats.
It’s not that they feel sympathy for their cattle-class passengers. If you’ve ever flown in cattle class you know they don’t. It’s just that they’d rather not have to clean up the mess.
I have flown in business class on the very rare occasion. It’s a different universe up there. The airlines recognize your basic humanity in that universe. And you typically get a washroom or two dedicated to your fellow business-class humans. And the placement of that washroom doesn’t require that you walk to the aft of the plane.