Here's the link to a tutorial at Wrigley Video Productions which demonstrates how to create a moving track matte in Premiere Pro.
In the Mograph wiki a track matte is defined as "an element used in Adobe After Effects or Premiere Pro to mask out portions of a layer. The masking is calculated by either the alpha channel or luma values of the element used as a track matte." Track mattes simply the process of combining different layers of video. That operation is commonly referred to as compositing.
While the tutorial demonstrates how to use the track matte to blur the face of the athlete, track mattes have many other uses. One common application is to use Premiere's titling program to create a text track matte, and then fill the text with another video layer.
That's all much easier to grasp by watching the video tutorial. Have fun!