Frames-per-second is the rate at which film is exposed in a camera. Silent films were shot and projected in a range from 16 to 18 frames-per-second. With the birth of the sound era, it became necessary to shoot and project at 24 frames-per-second, in order to produce clear sound on the optical soundtrack.
Changing the number of frames-per-second can result in a variety of effects. For instance, shooting a scene at 48 frames-per-second and then projecting it at 24 frames-per-second creates slow motion, since each frame is shown at twice the length of the original.