Offset is the location of the face of the wheel, compared to the center of the wheel.
So lets take a 10" wide wheel for example. It will actually measure out to be 11" wide (all wheels are 1" wider than the advertised size due to the barrel). That means the center of the wheel is at 5.5". If the wheel has a +20mm offset, that means the face of the wheel is 20mm out from the center of the wheel. If it was a -20mm offset, this would mean the face of the wheel is 20mm in from the center of the wheel.
