It's just until now it was hard to do logistically. How does one teacher with thirty students meet the needs of every student? And that's where we try to come in. If you need to learn something, if you're having trouble with it, keep working on it until you master it and then you go to a more advanced concept.

An explanation by Sal Khan about how Khan Academy, a non-profit organization that offers free educational content online, facilitates mastery learning.