Masking is a very useful technique in Flash, employed to change the shape and/or surface of any object. By applying a mask on a movie clip, you can restrict the visible area of the target clip to the masked area.

To apply a mask is very easy in the Flash editor:

Step 1

Open a new file in Flash editor. Draw whatever you like as the target clip on the main stage.

Step 2

Insert a new layer above the orginal layer. Draw your mask shape there. Right click on the new layer and select "mask" in the pop up menu.

That's all there is to it. Very simple. Good luck.