So on a rim, when bolted onto the vehicle, you have an inside edge and an outside edge (looking at it from the front of the vehicle).
The backspacing is simply the distance between the inside edge of the rim, and the part of the rim that bolts to your hub, typically measured in inches.
The LESS backspacing on a given rim, the MORE your tire protrudes from the vehicle's wheel wells. With larger-than-stock tires, you will want a smaller backspacing to avoid the tires rubbing on control arms or springs, etc.
The most common backspacing I see on after-market 4x4 rims is 3.75", which is often more than enough considering your stock rims' backspacing are usually upwards of 5".
Does this help?