The tiller is what steers a boat — specifically, the handle attached to the rudder. Tillers are generally found on smaller boats because it would take too much force to steer larger ships with hand tillers.
If you have never been on a sailboat before, it may take you a few minutes to figure out how to move the tiller to smoothly steer the boat because you might move the tiller in the opposite direction that you want the boat to go. If you want the bow of the boat to go starboard (right), for example, you need to move the tiller to the port side (left) and vice versa. Rapid back and forth movement on the tiller helps create drag and slows the boat.