The offset determines how far the center of the wheel is moved from the hub face (how far the wheel tucks inside the fender, too close it will rub the strut)
The centering rings are if you wish to retain hubcentric wheels. The center of the stock wheel fits over a round boss on the hub, this centers the wheel & holds the load (as opposed to relying solely on the lugs for this)
Aftermarket wheels are generally made with much larger through-holes so they fit a wide variety of vehicles, and thusly you'll need an adapter ring to retain hubcentricity