Here's little show and tell.
This is the back of the Fuel wheel with a 106.1 hub bore, which is the only one available for the wheel size I need.
However the hub bore for our trucks is a much narrower 77.8, so you need the ring to fill the gap, and the aluminum one looks like this:
You just slide it into the wheel before putting them on and the wheel sits perfectly in the hub:
By doing this all the load is transferred directly from the wheel to the hub. If you didn't do this all the load would be on the lugs holding the wheels in place in every direction.
Make sense?