A couple of weeks ago, Microsoft announced their plans to shift the Windows Insider Program from a ringed approach to one using channels instead.
As they stated at the announcement, the intent was to shift away from the rings representing the pace of its released builds to one that focus on quality. In addition, the channels also indicate where Windows 10 is in development with the Dev Channel indicating the early stages of development, Beta Channel in the focused zone of updates as features are locked down, and the Release Preview Channel representing the final stages of development with a focus on bug fixes and stability.
If you already have your test devices enrolled in Fast, Slow, or Release Preview Ring there is no action required on your part. As of today, those devices will be shifting to Dev, Beta, and Release Preview channels respectively.
Windows Insider Program Channels Screenshots