You create animation by changing the content of successive frames. You can make an object move across the Stage, increase or decrease in size, rotate, change color, fade in or out, or change shape. Changes can occur independent of or in concert with other changes. For example, you can make an object rotate and fade in as it moves across the Stage.
There are two methods for creating an animation sequence in Flash: frame-by-frame animation and tweened animation. In frame-by-frame animation you create the image in every frame. In tweened animation, you create starting and ending frames and let Flash create the frames in between. Every frame where something on the Stage changes is a keyframe. Frame-by-frame animation increases file size much more rapidly than tweened animation.
For an interactive introduction to animation, choose Help > Lessons > Lesson 7 Animation.