offset is how far the wheel's mounting point is off the centerline of the rim itself.
think of it this way. The more offset the further from the rim slides outwards toward the fender. You want this so that with the tire its close but not past the fender so the tire doesnt hit the fender.
Thats the most you can go and this will give you the biggest rear tire. I dont know that limit on the newer cars so cant tell ya.
for example on my car 3.75 inches from the rotor if you were to measure directly outwards that would be the limit where you want the rim lip to come out to so when you have the tire which should be a bit past that it wonthit the fender edge when the tire goes up towards the fender.
I have 10" wide rims in the rear with a 4.00 inch front spacing and a 6.00 back spacing so 6 inches of my rim are behind the rotor mounting surface and 4 inches are in front of it... as you can tell I am past my limit and had to roll my fenders
the newer stangs have rolled fenders from the factory so you can push a bit more... but that number I dont know.
read this http://www.mustangevolution.com/must...and-tires-faq/
here is a pic off offset explanation