Though it has served other purposes, the transom is the lateral "back wall" of a boat's hull structure, helping to transfer loads from the front and sides of the boat's hull to the keel, which is the backbone of the boat.
From the 15th through the 19th centuries, the transom was often fitted out as the quarters for the ship's officers and the ship's offices. Outer cabins had windows and were laid out to provide easy access to the main and quarter decks, which were the officer's primary duty locations.
The transom is the "square end" of the hull structure. As such, it provides a lateral bearing wall, transferring the forces of motion acting on the bow and sides of the ship, creating balance--much like the hips transfer the stresses of walking to the backbone.
The transom on a modern ship is large, as it must be, to transfer loading from the sides to the keel. On modern ships, the transom is likely to be an internal structure. On a yacht, the transom may be wide enough to form the wall of the salon. The transom of a small boat may only be large enough to carry an outboard motor.
The transom on a recreational boat may crack if too great a load is placed on it. This loading can result from too heavy a motor being mounted or from too powerful a motor imposing too great a load on the hull structure when operated at speed. Further, the length of the motor shaft should match the height of the transom.
Some recreational boats are equipped with transom doors, a pass-through allowing easy access to a stern-mounted swim platform. If you see even a small leak at the transom door, the problem should be investigated immediately, since a leak may indicate an internal crack in the structure of the transom that affects the integrity of the hull.