What Is Offset?
Offset is the distance between the wheel's hub mounting surface (where the wheel bolts to the vehicle) and the wheel's true centerline. On a wheel with zero offset, the hub mounting surface is at the wheel centerline.
If the hub mounting surface is toward the wheel face (the visible side of the wheel) side of the centerline, then the offset is said to be positive. This configuration places more of the wheel over the hub and brake components, and is common on front-wheel-drive cars.
Negative offset means the hub mounting surface is closer to the back side of the wheel, so more of the wheel extends outward from the hub. So called "deep dish" wheels tend to have a negative offset.
This diagram shows how different offsets would affect the actual positioning of a wheel
on a vehicle that is set up for zero offset.