Track mattes are an essential tool in creating motion graphics. At their most simple, a track matte is a still image/graphic/text/etc that is used to create a variable transparency for another layer in your composition.
So, if you want to show video framed in the font of your text, all you need to do is designate your text as the matte for the video.
This can be done on your Alpha Channel, or through your luminance channels. Alpha frames out everything under matte's alpha channels, while Alpha Inverted frames out everything outside of the alpha channels.
Or this can be done by asking After Effects to use the brightness of the top layer as the mask, this will give your mask an option for translucency.
Luma Matte will remove your brightest sections, while Luma Inverted will remove your darkest.