The centerbore is the large hole in the middle of the wheel that attaches to your vehicle's axle hub assembly. This allows the wheel to support the vehicle's weight, while lug nuts and stud bolts secure the wheel to the hub assembly. When installing a wheel, its centerbore needs to align exactly with the center of the axle hub as the lugs are torqued down. If the wheel is installed even slightly off-center, it can cause vibrations and shaking while driving.
Factory wheels are designed to fit on a specific make and model, so centering the wheel is usually not an issue. The centerbore on a factory wheel is tooled precisely to match the hub diameter for its intended vehicle, ensuring that the wheel is seated firmly on the hub and centered with the axle. This is called hub-centric fitment.
On the other hand, most aftermarket wheels have very wide centerbores in order to fit on many different vehicles. This means the centerbore tends to be larger than the hub. To remedy this, aftermarket wheels can be installed using hub-centric rings. A hub-centric ring serves as an adapter between the axle hub and the centerbore, filling the gap between them. This essentially increases the hub size so it matches the centerbore, allowing the wheel to be fitted and centered properly on the hub.
Without hub rings, the wheels must be centered by lining up lug nuts and stud bolts instead. Typically, the vehicle is lifted off the ground during installation to prevent its weight from pushing the wheels off-center. This is called lug-centric fitment. However, this puts more stress on the stud bolts, and can be especially dangerous if your vehicle was not designed for lug-centric wheels. Generally speaking, hub-centric rings are safer and more effective for installing new aftermarket wheels.