Due to all the issues with the previous Windows 10 feature update (1809), Microsoft isn't taking a risk this time. The rollout will be slower and users will receive more control over when and if it gets installed.
Among other things, this new Windows 10 release add a light theme, a Windows Sandbox to easily run software in a virtualized environment, some changes to the Start Menu, the ability to remove more built-in apps, a separation between the search bar and Cortana, variable rate shading support in DirectX, and a lot more.