This is Offset Curves, from Week 6: Curves.

This video shows you how I use animated graphics to demonstrate geometry. It's one thing to describe an idea in words, but it's often much better to show it! Some of the ideas here may be unfamiliar to you right now, but by this point in the course they'll all be old friends.

As always, the course materials contain this program, so you can run it yourself, and study it to see exactly how this technique works and adapt it for your own uses.