A 'bit' is a small metal stick that goes into a horse's mouth which helps control the horse more easily. There is what part of a horse's mouth that has a big gap where there are no teeth and this is where the bit is placed when riding. There are many different kinds of bits and they are usually in two pieces that are connected together by a smaller piece. (Sometimes it's all one piece with a dip in it of the shape of a 'U'). All of that is in the horse's mouth but connected to that is an 'O' or 'D' shaped ring on either side of the bit that is outside of the mouth and has the reins connected to it. Other than the reins, the horse wears headgear called a bridle.
The bit works by allowing the rider to turn the horse's head by pulling on the reins. With this, the rider has much more control over the horse than a bitless bridle. Having the ability to turn the horse's head while riding also gives the advantage of turning the whole body.
Not all the time can a bit be good for a horse. If it's used wrongly, it can give much pain to a horse and could injure its mouth, lips, tongue, and especially its teeth very badly. When a horse is made to do a particular type of work (like reining), the bit may be different from other bits. English and Western bits may also be different as well.
The first bits were made of rope, horn, bone or hard wood. Metal bits became in use between 1300 and 1200 BC. Sometimes bits are made of rubber, plastic or mixed with other metals.