Think of it like writing a paper:
Alpha - Rough draft, it's missing a bunch of stuff, there's typoes everywhere, you're not going to show that to anyone except maybe people who are helping you write, it's not even close to done.
Beta - First draft, it's mostly done, you still need to do some proofreading but you might share it with the class for some feedback before you're done. In this case, Runey shares the beta with patrons so that he can get feedback to make some last minute changes and bugfixes and the patrons get to access the content early - with the understanding that it's not done so there may be bugs and there may be changes before the final release.
Release - You're ready to turn it in to be graded. You might still work on writing more of it, you might even make revisions to hopefully improve your grade, but this version is at the very least good enough to hand in.