Yesterday Microsoft released Windows 10 Insider Preview Build 17101 for PC to insiders in the fast ring. As this release is more a bug fix for the upcoming release of RS4, there are not a lot of new features added. The highlighted features include emoji improvements, a new power plan for Windows 10 Pro, and updates to the out of box experience. As RS5 continues to be developed, insiders will find these builds becoming buggier as more features are added. Also included with this build is the Windows App Preview program. This program allows insiders on all rings to preview upcoming versions of certain apps.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-insider-build-17101-brings-emoji-changes-and-windows-app-preview/