Offset is a pretty simple concept. Imagine looking at a wheel from the front, looking through the tire. The wheel has a certain width, for argument say it is ten inches wide. The offset refers to where the mounting face of the wheel(the flat surface that goes against the axle) is located in relationship to the center line of the wheel. So on a 10" wheel, the center line would be 5" in from the edge of the wheel. Negative offset is when the mounting surface moves closer to the inside of the wheel and positive offset is when the mounting surface moves toward the outside of the wheel. 1" of negative offset would put the mounting surface 1" toward the inside edge of the wheel from the centerline and 1" positive offset would put the mounting surface " toward the outside edge of the wheel from the centerline. Hope this helps and I explained it simple enough.