Masking allows you to hide part of an object or text. It can be used for making spotlight kind of effects.

Step 1
Open a new file. Modify>Movie and make the stage 550 by 100. Set the background to any color you want. Rename the layer to mask. Insert a Keyframe at frame 40.

Step 2
Add a new layer and name this layer text. Using the text tool , insert some text into the first frame of this layer. I recommend a thick font like Arial Black, size 48.

Step 3
Now, click on the first frame of layer named mask, and draw a circle slightly larger than the text. Convert this circle into a symbol and place it on the left hand side of the text.
Click on frame 40 and Insert>KeyFrame and then pull the circle to the right side of the text.
Create a motion tween between frame 1 and frame 40 of layer mask. (Right click on the mask layer, choose create motion tween.

Step 4
The last step is to create the mask. Make sure the text layer is the top layer!!
Right-click on the top layer (text) and select Mask from the menu. Play the movie to see the results.

Note: The color of the masked object is set by the color of the masking object. In our case, the color of text is determined by the color of the circle. So even if you've used black for text color, the final color on masking will be the color of the object used to mask the text.