Windows 10 build 19551.1005 is the fourth Insider Preview build of Manganese, the 20H2 development semester of Windows as a service. This build was released to Windows Insiders in the Fast Ring on 23 January 2020 and has general changes, improvements, and bug fixes which are listed in the official release announcement.
Windows insider received a new build 3 days ago in Fast access channel. The update carries fixes for a few issues, but there are still some problems in the list that have not been resolved. You can read the changelog. 2b1af7f3a8