A horse curb strap is a piece of equipment used in horse riding to attach to the curb bit of a bridle. It is typically made of leather or chain and goes under the horse's chin to help stabilize the bit and provide additional control during riding. The curb strap also helps to prevent the bit from sliding through the horse's mouth. It is important to adjust the curb strap properly to ensure that it is not too loose or too tight for the horse's comfort and safety.