"What are these plastic (or metal) rings that came with my wheels?"
"Why didn't my wheels come with hub centric centering rings?"
The terms "hub centric" and "lug centric" describe the method used to center a wheel on a vehicle's hub to prevent vibration. Most Original Equipment wheels are built with the same centerbore diameter as the hub diameter on the vehicle so the wheel is centered as soon as it's sitting on the studs or the hub. Vehicles have many different hub diameters (or hub bores), therefore aftermarket wheel manufacturers generally make wheels with a large centerbore so the wheel will go on many types of applications. They either use a centering ring or conical seat lug hardware to center the wheel on the hub.
A centering ring (or hub centric ring) can be made of metal or plastic and either clicks into the back of the wheel or slides onto the hub of the vehicle - that way when you torque the lugs down, the wheel is already centered. The other way to do this, which is more prevalent on truck and SUV fitments, is to use lug holes with cone-shaped bottoms with conical seat lug hardware. When installing lug centric wheels, jack the axle up so the wheel and tire are off the ground, then firmly hand-tighten the lugs down so the wheel meets the hub. Then, gradually and evenly start applying torque a little bit at a time to each lug in a star-shape pattern. As you add torque, the cone-shaped lugs shift the wheel upwards on the hub and center the wheel properly, preventing wheel vibration.