What are reins used for on a horse?

The reins are long straps of leather or soft rope that are connected to the bit (which rests in the horses mouth) at one end and are held by the rider at the other end. The reins are used to guide the horse's direction of travel - straight, left, or right.

In English style riding, the reins are attached directly to the bit at the level of the horse's mouth and are gently tugged to turn the horse's head one way or the other. In Western style riding, the reins are generally attached to downward extensions of the bit called the shanks, and the reins are laid against the sides of the neck to turn the horse one way or the other - the reins are only pulled on when correcting the horse or getting its attention.