Wheels come in different diameters and widths. OFF-SET
basically means, the distance from its hub mounting surface to the centerline of the wheel. Along with the wheel width, this will determine how far the outer portion of a wheel and tire sticks out.
You've seen wheels and tires that are even or flush with the edge of the fender well. That's acheived not only with a wider tire and or wheel, it's also done by selectively choosing a wheel that has the correct OFF-SET
from the mounting point.
The best way to see what OFF-SET
is and how it works is, go to a local wheel and tire shop that sells custom wheels and ask them to show you how OFF-SET
Here is a website that may explain it better.