Bridle

A bridle is a piece of equestrian headgear used by a rider to control a horse while riding it. A bridle consists of reins and a bit. The reins are long ropelike strips held by the rider, which are in turn attached to each side of the bit. The bit is a piece of metal which is placed in the horse's mouth.
Usage

Pulling on the reins can be used to steer or stop the horse. The sides of a horse's mouth are sensitive, so pulling on the bridle pulls the horse's head from side to side when the bit is pulled, which is how the horse is controlled.
See also
Horse tack