Stands for "Frames Per Second." FPS is used to measure frame rate – the number of consecutive full-screen images that are displayed each second. It is a common specification used in video capture and playback and is also used to measure video game performance.
The term applies equally to film and video cameras, computer graphics, and motion capture systems. Frame rate may also be called the frame frequency, and be expressed in Hertz.
More frames per second means smoother motion. Low FPS means motion will appear "choppy".
24-30 FPS is generally considered good - it is the level at which motion will appear smooth to most people. 60 FPS video is also common and may appear smoother. Some devices can display 90 or higher FPS, most commonly for games.
Phones with a slow-motion video feature typically capture at higher FPS rates (120 and higher) and then play back that video at a normal viewing FPS (30-60) in order to create the smooth slow-motion effect.