WebRender is an experimental GPU rendering back-end for Servo. WebRender tries to offload as much of the rendering work to the GPU rather than having to draw the web content via the CPU.
This back-end is still very experimental and only enabled if launching Servo with the -w switch. Nevertheless, it's a huge milestone that WebRender is now merged.
If you are interested in learning more about this GPU accelerated back-end for Servo, see this Wiki page.