https://wccftech.com/windows-10-19h1-build/

Microsoft has just dropped a new Windows 10 Insider Preview build to both its Fast Ring and Skip Ahead (we hear only a few spots are left) Insiders. Windows 10 Insider Preview Build 17723 from Redstone 5 RS5_RELEASE branch is available for those in the Fast Ring. Along with that, Windows 10 Build 18204 from RS_PRERELEASE branch (hello, 2019 Windows!) has been dropped for those who managed to get into the Skip Ahead program.

While right now both are bringing the same changes and improvements, Microsoft is expected to move to the Windows 10 Spring 2019 Update codenamed 19H1 (Redstone codename is apparently going to die) in a few weeks.