A centerbore is the machined circular hole on the back of a wheel.  Its purpose is to ensure that the wheel fits perfectly centered onto the vehicle's hub.

 

Centerbores on most OEM wheels come machined to exact specifications for the vehicles they come on.  Aftermarket wheels usually have a standard centerbore measurement, which may or may not fit your vehicle without a small plastic adapter called a hubcentric ring.

 

Should you ever move your wheels to a different vehicle or need replacement rings for any reason, they can be obtained at: