A pinion snubber is a bumpstop for your rear end that prevents contact between your rear end housing and the body, and/or contact between your driveshaft and the body or any underbody components it may contact should your rear suspension become compressed further than intended. The snubber (bumpstop) is mounted to your body and will contact the pinion end of your rear end housing under compression. I do not run a snubber, but it is not a bad idea to have one.