Leads refer to the front leg which a horse reaches out further with as it rounds a bend at a canter or gallop. When a horse is rounding a corner he will be better balanced if his inside front leg is leading.
If a horse is going in a right hand circle he will need to lead with the right foreleg. The left foreleg will take a shorter stride. The footfall pattern of a left lead will be a sideways mirror image of a right lead.

