Microsoft has made some big changes to the Windows Insider Program over the last year and a half. It started in January 2019 when Insiders started testing out Windows 10 20H1 around eight months earlier than they normally would. The head of the program Dona Sarkar left in October, and then there were big changes in December.
That's when Microsoft announced that it was killing off the Skip Ahead subset of the Fast ring and that it was changing how builds were delivered. From then on, Fast ring builds were said to not be tied to a specific Windows 10 release, and the Slow ring was dedicated to servicing a release before it could be promoted to the Release Preview ring, and then production.
Now, this long process of evolving the Insider Program is continuing. First of all, rings are becoming channels, similar to what we're already seeing on other Microsoft products like Edge and Office. Secondly, the Fast ring is becoming the Dev channel, the Slow ring is becoming the Beta channel, and the Release Preview ring is becoming the Release Preview channel.
The actual content of the Windows Insider channels isn't any different from what we were getting from the rings. You'll see prerelease builds from Dev, more reliable builds that are tied to a feature update in Beta, and then Release Preview will continue to offer builds that are slated to be released to the public at some point.
The whole idea is that with the way that the program has evolved already, the old names just don't make sense anymore. The old names imply a frequency of build releases, rather than reflecting quality. That's what the new names are meant to reflect.
Microsoft says that you'll start seeing these changes later on this month, and you'll automatically be moved over to the channel that corresponds with the ring you're on.