Hub Centric vs. Lug Centric
 

 
This is the brake/ hub assembly with the wheel removed.  I've highlighted the hub red to make it more obvious.  The stock wheel is usually hub centric, which means the center of the wheel fits snugly exactly to the diameter of the hub.    

 

 Many aftermarket wheels will have larger diameter hub openings.  If you have matched the bolt pattern, you could just bolt the wheel in place, but you would be relying on the lugs to center the wheel (a lug centric set-up).  To make your aftermarket wheel hub centric, you can get adapter rings to increase the size of the hub.  This is the assembly with the adapter in place.  The red dot shows where the stock wheel would rest, and the blue dot shows where the aftermarket wheel rests.  The plastic ring converts the wheel/hub to the more desireable hub centric set-up.