A bit is a type of equipment that is put in a horse's mouth to help control it. It is attached to a bridle to hold it on the horse's head, and to reins which the rider holds and allows them to control and cue the horse. The mouthpiece of the bit is the part that goes into the horses mouth. It is usually made of metal, but may also be made of plastic or rubber.
There are many different types of bits, including snaffles, pelhams, and curb bits. They all work a little differently. Some bits are "softer" than others, meaning that a pull on it is gentler to the horse than a "harsh" bit. For example, bits with a smooth mouthpiece are soften than those with a twisted mouthpiece. Bits are not cruel when used correctly, but a rider that pulls too hard can hurt a horse's mouth, especially if they use a harsh bit. If a rider is inexperienced, they should ride with a softer bit so when they make mistakes they do not hurt the horse. Experienced riders who can follow the horse's mouth as the horse moves and give very slight cues can use bits that are harsher. They have the skill to use the bits for very fine control over the horse, without hurting it.
Certain bits are commonly used in certain horse activities. For example, western style riding usually uses a curb bit. English style usually uses a type of snaffle, although certain horses may be put in a different type of bit. Saddleseat riding and high levels of dressage riding use a double bridle. This is a bridle that holds 2 bits. Each bit works a little differently so the rider can cue the horse to do different things. The rider must ride with 2 reins in each hand. This requires the rider to be skilled so they can handle both reins without confusing the horse.