What does it mean for a course to have a certain amount of rubies?
Users are free to donate their rubies to courses. When a course receives rubies, the author does not get them. Instead rubies currently help us determine which course should be considered for being "featured" and in the future, rubies will be used as an automatic ordering mechanism on the site.
In a way, it's yet one more different way to show how well recieved a course is.