Hi!
Let me start out with saying that this is a truly wonderful course with very thorough walkthrough of all parts of GIMP. I enjoy it very much.
I must, however, come with a small complaint regarding the selection tools and “feathering”. I believe that you skipped over this feature a little too fast, and the short description of it was a bit vague. 
What feathering actually does is make the edge of the selection “fuzzy”, which means that as you go nearer the edge, the selection will slowly fade to nothing. The radius defines the distace from the selection edge that the selection is either full or nothing.
I realize it is hard to describe with words, but here is an image I made while following the lecture:
What I did was I had a black “mouth” shape. Then I selected the shape with fuzzy select, and feathering set to 8 pixels. This created a selection around the “mouth” that is “fuzzy” at the edges. It is hard to see this by looking just at the selection line. Therefore, I picked the pencil tool again and painted everything with blue. The result is that the middle of the selection is completely blue, but as you come closer to the selection edge, the blue has less and less power. It goes towards black (since the mouth is black), and then, at the mouth “edge”, it starts out as light blue since the blue color is “half-painted” on the white background. The blue then continues to fade until it is white.
Also, regarding feathering, it is possible to add a “fuzzy” (feathered) edge after the selection has been made. This is done through the menu “Select” -> “Feather”. There you can specify the radius of the “feathering” of the selection.
Hope this makes sense.
Best regards
Carl