windowscentral[.]com/microsoft-changing-testing-windows-10
After you sign up for the Insider program using a Microsoft account, with each copy of Windows you want to take part in the program, you can select one of three modes or rings, Fast, Slow, & Preview.
The Fast mode usually got 1 - 2 versions to install & try out per week -- while not Alpha builds, they've focused on the newest, least tested, least reliable new features &/or fixes. The Slow ring was closer to a regular Beta build, with sometimes months between new builds that were reliable enough to use everyday, if you didn't mind the occasional glitch. The Preview ring got more-or-less release candidates, testing for problems using a build that was complete & generally ready for the masses.
And up until now, regardless which ring you chose, taking part in the Insider program let you see what was coming to Windows 10. While the fan boys might take part thinking it's cool, for devs &/or anyone managing a couple or more Windows devices it was a heads-up, giving you a chance to spot potential problems as well as expected benefits. What changed is that the Fast ring may not be beta testing the next version of win10 at all. It seems that anything Microsoft wants to test for free, it's going to stick in the Fast ring, IMHO removing pretty much any potential benefits to users. Needless to say, I've just switched my VMs from the Fast to the Slow rings.