Frames per second.
It's the number of frames which appear per second, as per the old cine films.
You need at least 25 - preferably 30 - frames per second to achieve smooth motion.
The brain fills in the "missing" frames to achieve the sense of smooth, rather than jerky motion.
If you watch a cartoon film, it's made up of thousands of frames with the characters being moved very slightly between each separate frame. When shown at normal speed the eye sees it as normal motion.