What you are doing (or simulating doing) is pulling ropes to rotate the boom, the horizontal pole attached to the mast, the vertical pole. Since the sail is attached to the boom, the sail rotates too. This changes what's called the sail angle. Moving the sail away from the center of the boat is called trimming the sail out, moving it back towards the center of the boat is trimming the sail in. Why do we care about the angle of the sail?