The idea of timeline animation goes back to flipbooks. If you ever made a flipbook you were doing frame by frame animation. Each frame appears on the screen for a fraction of a second. Typical frame rates are from 15 to 30 frames per second. By using 15 frames per second timing, you are making each frame visible for 1/15 of a second. Beyond about 30 frames per second, the computer cannot display more information and your eye stops being able to see the difference.