You can think of layers as transparent sheets of acetate stacked on top of each other. Where there is nothing on a layer, you can see through it to the layers below. See Changing the order of layers.
When you create a new Flash movie, it contains one layer. You can add more layers to organize the artwork and animation in your movie. The number of layers you can create is only limited by your computer's memory, and layers do not increase the file size of your published movie.
You can draw and edit objects on one layer without affecting objects on another layer. In addition, you can use special guide layers to make drawing and editing easier, and mask layers to help you create sophisticated effects.
Use separate layers for sound files and actions. This helps you find all your actions and sounds quickly when you need to edit them.
For an interactive introduction to layers, choose Help > Lessons > 4 Layers.