Windows Central has details on what to expect. It will be the biggest change to Windows 10's user interface since the operating system was introduced in 2015.
Microsoft's goal is to "reinvigorate" and modernize the Windows desktop experience, with updates coming to many top level user interfaces like the Start menu, Action Center, and even File Explorer. Windows 10 developers will work on offering consistent modern designs, better animations, and new features. It will reportedly be an evolution of Fluent Design, inspired on Windows 10X, and not a complete redesign.
It's still too early to nail down exactly what will be updated with Sun Valley, but sources have said to expect new Start menu and Action Center experiences, likely based on those same experiences found on Windows 10X, but tailored for desktop. Microsoft is also working on an updated Taskbar built with modern code, and an improved UI for the legacy File Explorer.
For tablet users, I'm told that better animations and a more "fluid experience" is on the cards. We already know that Microsoft is redesigning the touch keyboard and emoji picker, as those changes are already live in the Windows Insider Dev Channel. Microsoft will also continue its escapade of rounding-off corners throughout the UI, including app windows and other shell areas.