Up until about 1000 AD, boats were steered with a rudder (usually an oar), that had to be held by the sailors on board. By doing so, they would push the water to the sides of the boat in order to steer it. In Ancient China, however, they saw the introduction of the stern-mounted rudder.
This rudder was built into the ship rather than held, and would be controlled by a stick attached to the rudder, which was now a large rotating board mounted on the back of the ship, or later angled through the use of a steering wheel on larger ships.
This reduced the amount of people needed to man a boat in order for it to sail, and enabled the existence of single-person small sailboats, bringing us closer to modern sailing.