Tidal Lock refers to the condition in which an object has the same rotational period as its orbital period. As a result, the object always shows the same face towards its orbital center. The cause of tidal lock is tidal drag or tidal acceleration, which is a force created in an object spinning at a different rate from its orbital rate due to the bulge (tidal bulge) which forms in the surface of the object due to the gravitational pull of the orbital center.
In an object where the rate of rotation is different than the orbital period, the tidal bulge moves across the planets surface. The tidal drag results from mechanical resistance in the the surface material as it is being distorted to form the bulge and later as the bulge collapses. The energy which supplies the force necessary to overcome the surface's mechanical resistance comes from the rotational energy of the orbiting object itself.
Over an extended period of time, the tidal drag will cause the orbitting object to slow its rotation period until the tidal bulge of the orbitting object becomes stationary relative to the object's surface.