Offset is important b/c it essentially tells you how much tire you can fit in a certain space. It does alot of other things, but your prob is with tire fitment. If you could pretend that where the spindle bolts to wheel would be an invisible line parallel to the earth with zero camber. The offset of a wheel can be a positive or negative relation to where that invisible line runs. That's kinda a shitty way of explaining it, but I'll see if I can't find a site with a good pic explaining it. I use to cheat that offset to use a wider rear tire in some of our older muscle cars so that we wouldn't have to tub a rear end. Drift cars mess with offset to run wider tires but also run real cool set-ups to allow the camber work for them.

Edit: I was way off. Stock offset is 46mm.