Two different measurements.
Offset is measuring from the center of the wheel to the surface that touches the brake hub/rotor when that wheel is bolted on. It could be positive or negative.
Back space is measuring from the back edge of the wheel to the same surface that touches the brake hub/rotor when that wheel is bolted on.
From what I've heard, back spacing is pretty much what you want to be dealing with when looking for wheels for the TJ. What your stock Moab wheels had I do not know.