This is a new Linux explicit synchronization protocol based on DRM synchronization objects 1.
Since nvidia drivers do not properly implement implicit sync, this protocol not existing is the root cause of flickering with nvidia graphics on Wayland. This MR being merged means that Wayland might finally be usable with nvidia graphics with the next driver release.
God I fucking hope so man. I'm so tired of Nvidia not working on Wayland properly. It'll be so nice to have VRR and gSync on Wayland without the awful flickering.