offset or backspacing = the distance from the back surface of the wheel´s hub (that mates to the brake disc/bearing hub) to the center of the wheel. In other words, if you have a 6" wide rim, the center is 3" in from either rim. Nearly every single FWD car uses a positive offset (wheel hub is closer to the outside of the wheel than the inside) of 20-50mm.
The offsets on the scort wheels seems to run between 38mm for smaller rims and 45mm for the larger rims (since the rim is large enough in diameter to miss hitting some suspension peices). If the offset number is too high you can add spacers. If you add a 5mm spacer to a 50mm wheel you´ve essentially made a 45mm backspacing.
Another thing to consider (which most dont), is the diameter of the lip on the bearing hub that the center of the rim fits around. Most aftermarket wheels do not properly match up with that rim.. they are usually made larger. The thing is, the lug bolts on a car are not supposed to support the weight of the vehicle.. that center lip is. If the wheel isn´t ´hub-centric´ you can run the risk of the wheel shifting out of balance at high speeds or shearing off your lug bolts.. not likely, but important to consider.
PS: I took the 13x5.5" 13mm backspaced alloys off my BMW and bolted them up to the scort for a run around the block a few months ago. They were nearly perfectly flush with the front fenders.. I didn´t even try them in the back because I was pretty sure they´d hit the fenders.