Major Nelson confirmed that Variable Rate Shading will be a feature on the Xbox Series X.
What Is Variable Rate Shading?
To put it simply, VRS essentially affects how hard the GPU works when shading or rendering certain areas of a frame.
VRS will basically increase rendering performance and quality.
What Does This Mean For Xbox Series X?
With Variable Rate Shading, developers no longer have to shade every pixel individually. Now that gamers are jumping to 4k (and at some point in the future, 8k) consoles and GPUs would usually struggle to keep up.
However, VRS can help speed up game engines just by using it for pixel shading, it's essentially future-proofing our consoles.
If you'd like to get a really in-depth look into how VRS works, you can check out this article explaining it better than I ever could.