Windows Insider enrollment has been opened again for Windows Insider who want to access the first build of Windows 10 19H2, the second big update of the year. Windows Insiders part of the Fast and Slow rings will have access to the April 2019 Update builds only. The first build is not expected to include tons of new features as the work on the next big update has just started. Microsoft will continue to spend more time on finalizing the Windows 10 version 1903 (April 2019 Update) which is expected to come out in April.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-opens-up-skip-ahead-ring-as-it-prepares-first-windows-10-19h2-build/