Intro
Masking is used to show and hide portions of an image/object. Imagine a box with a whole on top. The hole is the mask, and the content in the box is what is being masked. You can only see what the hole allows you to see. This is generally what masking is. Masking is often used as a way to make objects fade in and out of view. This tutorial shows you how masking works.
Step 1
First open a new document and draw a circle. Press F8 and convert it to a graphic symbol.
Step 2
Now create a new layer and insert an image you want the mask to affect. If you don't know what masks do, then just insert any image. Right click layer 1(the circle layer), and select Mask. Now, click and drag layer 2(layer with image) into/under layer 1(layer with circle). The layer icons should now both be blue.
Step 3
Goto the layers menu and unlock the layer which may have been locked upon changing the layer to a mask. Now on layer 1, make sure the circle is directly left of the image. On the same layer, enter keyframes at frame 20 and 40. On layer 2, insert a keyframe at 40.
Step 4
On frame 20 of layer 1, move the circle directly across to the right of the image. On layer 40, move it back to the left(it should already be there, if not move it).
Step 5
Now insert motion tween between frames 1-20 and 20-40 on frame 1. Lock both of these layers and press Ctrl+Enter to view your mask in action. It is very important that you lock both layers or the effect will not work.
Final Result:
More Flash & Swish: 
