Hub centric rings make the rims with even a larger central hole sit firmly and the wheel rotates without bouncing.
The package contains 4 wheel hub centric rings, ie when you order one set, you get 4 adapter rings.
The hub centric rings are sorted according to the car's wheel hub (inner diameter of the adapter ring) and from these you can select a suitable rim center hole (outer diameter of the adapter ring).
Center cups for rims can be found here: rim hubs