It's totally free. Teachers can request free private repositories at https://education.github.com/discount
Maybe a video showing the creation of an assignment and sending it out (url via email, is it?), students completing the assignment, and teachers looking over a completed assignment.
If that is a poor example of what this does then I missed the point entirely.
That's rather unfortunate, as more and more of the people putting their degree to work definitely need to know what it is, especially in start ups.