They basically are a ring of metal which pushes the wheel away from the hub mounting face, they come in various sizes, and in this game are there purely to make wheels give the impression of filling the arches, and make it look wider than it actually is.
You will need longer wheel bolts so that they will get through the wheel into the hub via the holes on the spacers. Type and size you need will depend on the ET of the wheel, and how much you want/need to push them out by.
