Chix - if i remember correctly the lesson revolves around C, G & D - but if you keep the shape you can play it anywhere and it becomes the same chord as the root note for each position - eg if you played the first shape Justin shows with the root on fret one you have an F chord, move the shape to the second fret it becomes F#, move it to the 3rd fret it becomes G and so on.
I think what Justin is doing is imploring you to become familiar with each root note for each shape - not just G, C & D.
Cheers, Bootstrap