The plain, jointed snaffle is the bit in which many horses begin their education. It's the most basic design of bit, with a single joint in the center of the mouthpiece.
When pressure is placed on the reins, the mouthpiece breaks at the join, forming an angle which gives the tongue room. The mouthpiece acts on the lips where they cover the bars of the mouth.
Some horses with a low palate may go better in a multi-jointed bit, such as a French Link snaffle.