The offset is how far in or out your wheels are from the hub. If they are too far in (too high?) they'll rub on the suspension, although if the offset is too far out (too low?) they'll stick out too far, and depending on how low the car is you might have to worry about the tires rubbing on the fenders. It really only makes a big difference if you either have the car lowered a lot, or at looking to squeeze the biggest tire possible into the wheelwell. hope this helps.
I might have the offset direction backwards, i always get it confused :-\
i already have 17s but i duno if there to far out and might rub on the fender thats why i sold by B+G springs cause i think that might be to much of a drop i can take a pic of my wheel to show you what im talking about if that could help?