The leg-yield is a lateral movement performed on a horse, in which the horse travels both sideways and forward at the same time. The horse is fairly straight through his body in the leg-yield, although he may have a slight bend to the outside (away from the direction of travel). It is one of the "three initial movements leading up to true lateral work", the others being the turn on the forehand and the shoulder-fore.
Read more about Leg-yield: Difference Between The Leg-yield and Half-pass, Uses and Disadvantages of The Leg-yield, Different Ways To Leg-yield