The traffic light movie clip has 60 frames. There are keyframes in 1, 20, and 40. A keyframe is a frame where there is a change.
In frames 1 through 19, the light is green. In frame 20 the light changes to yellow and is yellow for frames 20-39. In frame 40 the light changes to red. It stays red until frame 60. After frame 60, it starts over again in frame 1, a green light.