When something's at the back of a boat, it's aft. You might instruct a passenger on your sailboat to move aft while you adjust the sails.
Aft, which can be used as an adverb or an adjective, is almost solely used in nautical or aeronautical terminology — in other words, when you're talking about a boat, ship, or airplane. Flight attendants usually start at the front of the cabin and move aft as they dole out drinks and pretzels, and they might instruct you to stow your huge bag in an aft compartment. The Old English root is æftan, "from behind or farthest back."