
The goal here is to reduce system power consumption under low loads. For example, when you're doing 2D work like typing an e-mail or browsing a webpage, Windows 11 will reduce the refresh rate of your screen. This feature will work dynamically, to ensure you retain a fluid desktop experience. It's similar to the variable refresh rate used on Android smartphones.
[T]he instant you start scrolling down a webpage or moving a window around, Windows will automatically boost the refresh rate back to 120Hz (or whatever your display supports), giving you a very fluid desktop experience. -- Tom's HardwareThe downside is that dynamic refresh rate has various requirements. You will need a screen with adaptive refresh rate support, like FreeSync or G-SYNC, an it needs to support at least 120Hz refresh rate. Furthermore, it will only work if your video card supports WDDM 3.0. Additionally, support needs to be added on a per-application basis, so it requires a lot of software work for broad support.
At the moment, it's unclear whether Microsoft will push this exclusively for laptops or whether it will also be rolled out for desktop PCs.