Flash animation is based on frames. Each frame can have a slightly different picture.
These pictures are displayed one at a time, the same way cartoons and movies are done to create motion.
Most Flash movies use 24 frames per second, but you can change this.
The Flash developer can work with the timeline to insert, delete and modify the frames.
The frames where something changes are called keyframes.
In the first example, the flashing light, the light is black in frame 1.
In frame 10 there is a keyframe (indicated by a black dot) and the light changes to red.
The movie has just 20 frames.
Frame 20 is just a regular frame (indicated by a white square), not a keyframe.
After frame 20, the movie starts over again in frame 1.
In the second example, the dog with the wagging tail, the dog has just 8 frames.
On the top layer, the body, there is just one keyframe. The body never changes.
On the second layer there are 8 keyframes, in each frame the tail is in a slightly different position.
In frames 1-4 the tail moves down.
In frames 5-8 the tail moves back up. The first and last frames are the same.