Flash Templates Tutorials - Layers.
Each scene in Flash movie may consist of any number of layers.
Flash can not set various animation effects for several objects in a single layer - layers are needed in order to separate different but simultaneous animation assignments for each moving or changing objects.
It is important to remember that if you want to change differently the form or position data of multiple objects using tweened animation in a single layer, then there should be the Blank Keyframe between the keyframes of starting animated actions. And if you want to make different animated actions with several objects simultaneously, then each of them must be in a separate layer.
Typically, the background (usually the lowest) layer contains one or more static objects, and extra layers contain one animated object each. The layers in the time line are showed as lines. When the movie has several layers, do not try to work simultaneously with several objects from different layers - the work will become much more simple if at one time you will be editing objects (or its properties) located in one layer.