Premiere Pro and other nonlinear editors like it have a general operating practice: Clips in video tracks above Video 1 trump clips in tracks below them on the timeline. In other words, whatever appears on the highest track covers up whatever is below it.
However, the object isn't to use clips in tracks above Video 1 to obliterate what's beneath them. It's to use compositing to enhance what's down there. Premiere Pro gives you many ways to layer videos, graphics and images for best effect.
You use compositing techniques on clips so the clips below them on the Timeline can show through. There are four basic compositing methods:
Reducing the opacity of an entire clip.
Alpha channel transparencies in clips and effects.
Chroma, color and luminance keying effects.
Matte keying effects.
In the upcoming mini-lessons you will try all of these compositing methods and use different techniques with a few you've already tried. Once you see all the possibilities, you'll start to plan and shoot your projects with layered videos, graphics and images in mind.