Sailboats are propelled by the force of the wind hitting their sails, and further directed by the normal force the water makes on the boat's hull, keel, and rudder. The force of the wind on the sails is a function of the speed and direction of the air relative to the boat, known as the apparent wind, as well as the positioning of the sails.
Apparent wind can be thought of as the combination of two separate factors: the true wind, that is, the speed of the air relative to a reference frame (be it the Earth or the water), and the induced wind, which can be described as the wind that would be felt due to the motion of the boat relative to the reference frame if there were no true wind, that is, if the air were stationary relative to that frame. The induced wind has the same magnitude but is opposite in direction to the motion of the boat. Mathematically, we can state: