Short answers, pulled from the story.
Frame rate is the frequency at which consecutive images are captured or displayed, typically measured in frames per second (FPS) or hertz (Hz). It applies to film cameras, video cameras, computer animation, motion capture, and real-time rendering by a GPU.
24 FPS became the standard for 35 mm sound film between 1927 and 1930. When sound film was introduced in 1926, a fixed rate was required because the human ear detects frequency changes more readily than the eye. The industry chose 24 FPS as a compromise between the 22-26 FPS range many theaters were already using for silent films.
The flicker fusion threshold is the point at which modulated light, such as a computer display, appears stable rather than flickering. Studies show most people perceive modulated light as steady when the rate exceeds 50 FPS. When the light carries an image rather than a plain glow, the threshold rises significantly, into the hundreds of hertz.
Analog television frame rates were synchronized to the frequency of the electrical grid rather than inherited from film. Countries with 50 Hz power grids, covering most of the world, adopted 50 FPS; North America, Japan, South Korea, Mexico, Canada, and the Philippines, on 60 Hz grids, adopted 60 FPS. The color television era then lowered the 60 FPS standard by 0.1 percent to approximately 59.94 FPS to eliminate an artifact called dot crawl on legacy black-and-white displays.
Animating on twos means holding one drawing for every two frames of film instead of drawing a new image for every frame. At 24 FPS, this produces 12 distinct drawings per second. It is a cost-saving technique used widely in traditional animation; animating on ones, a new drawing per frame, is reserved for fast movements where twos cannot convey the motion adequately.
Screen tearing occurs when a game's frame rate and a display's refresh rate fall out of sync, causing the display to show parts of two different frames simultaneously as a horizontal split. Vsync prevents tearing by capping the frame rate to the display's refresh rate, though it adds input lag and can introduce stuttering. Variable refresh rate displays resolve the trade-off by dynamically matching the display's refresh rate to the game's current frame rate.