A mask creates a hole through which you can see the layers below. Masks in Flash are very similar to masks in Photoshop or other graphics programs. Imagine looking through a set of binoculars. You can only see through the lenses. As you move around, you can see other areas. A mask works the same way.
Open the following example saved as AdvancedAnimation/demos/Mask.fla.
Notice that the icons for the layers in the screenshot above are different than most layers. The circle layer is the mask layer. Also, the masked layer (flowers) appears to be indented as if it were in a layer folder. Any additional layers may be added to the masked section by dragging underneath the mask layer. To remove a layer from the mask, just drag it up or down.
Please be aware that if the layers are not locked in the Flash Timeline, you will not be able to see the mask on the stage. This phenomenon only occurs in the flash program, once the animation is properly previewed, or published, the mask layers work properly.
Any layer can be turned into a mask layer.