Microsoft officially kicks off the new Windows 10 Fast Ring with the release of their latest Insider build. Under this new Fast Ring, Insiders will always receive the latest code from Microsoft developers who are creating new features in Windows 10. Microsoft announced in November that they were no longer going to offer the Skip Ahead Ring and that all future preview releases containing the latest codes and features would be in the Fast Ring going forward. The Fast Ring will always contain the freshest code and newest features, but these features will only be released when they are ready, if at all.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10s-fast-ring-becomes-a-microsoft-dev-playground/