Premiere's Track Matte key type lets you customize and layer movies. When you apply this key type you can play one movie through the matte of another while yet another movie plays in the background.
When you create a track matte effect, the order of your clips is important. Applying the track matte key to different clips and different video tracks results in a wide range of effects. You'll see some of those effects in this lesson. In Lesson 10 you'll apply the Track Matte key type to a moving image and create a traveling matte effect.