The "Matte" is created by you. It could be from PrE's Titler, or perhaps from Photoshop/Photoshop Elements. It defines the areas that are "seen," and those "hidden," or Transparent, by the Matte.
Hope that helps,
So I don't actually have to create and store one? I could use, say, a Color Matte?
From what I have gleaned from the manual, this is essentially like a Gradient Layer in PhotoShop (which may be used in a non-gradient manner to allow selected portions of an image to show through another). Although it appears to be structured slightly differently.
The creation of the Matte, to be used with Track Matte Keying, will depend on exactly what one is doing.
Track Matte Keying is a very powerful tool, and can allow one to accomplish many things. This article points to but one potential use. The mechanics are the same, for other uses: http://forums.adobe.com/message/1765005#1765005
I have used Track Matte Keying to "soften" the skin on a young lady speaking, basically a "talking head." In that case, I did a duplicate of the Clip with the lady, and added a bit of Blur to that. Then, the Matte was created to only show the skin areas, with the lips, eyes, hair and background sharp. The Matte was then Keyframed w/ Position, to follow her head movements. That Matte masked out the Blurred areas for the lips, eyes, hair and background. As such it was not really a Shape.
Another use for Track Matte Keying is to allow for the superimposition of the display on a TV monitor, where the Clip on Video 2 is a PiP (Picture in Picture), and the Matte is an irregular rectangle, masking out all of the PiP, except for the area of that TV.
Lots of uses, and each will dictate what the Matte should look like.
Good luck, and hope that helps,
You can also find lots of great uses for the Track Matte - along with step-by-step, illustrated directions -- in my special effects book for Premiere Elements 11 and 12.