The Track Matte Key reveals one clip (background clip) through another (superimposed clip), using a third file as a matte that creates transparent areas in the superimposed clip. This effect requires two clips and a matte, each placed on its own track. White areas in the matte are opaque in the superimposed clip, preventing underlying clips from showing through. Black areas in the matte are transparent, and gray areas are partially transparent.
A matte containing motion is called a traveling matte or moving matte. This matte consists of either motion footage, such as a green‑screen silhouette, or a still image matte that has been animated. You can animate a still by applying the Motion effect to the matte. If you animate a still image, consider making the matte frame size larger than the project frame size so that the edges of the matte don’t come into view when you animate the matte.

You can create mattes in various ways:
Use the Title panel to create text or shapes (grayscale only), save the title, and then import the file as your matte.
Apply the Chroma, RGB Difference, Difference Matte, Blue Screen, Green Screen, or Non Red Key to any clip and then select the Mask Only option.
Use Adobe Illustrator or Adobe Photoshop to create a grayscale image and import it into Adobe Premiere Pro.