Backspacing is the distance from the back of the mounting hub on the rim to the back edge of the rim. On my S10, the wheels I have are made to order, the fronts are 8" wide with a backspacing of 4.5 inches so the rims are about 3.5 inches of space in the front. On the back I can run a wider rim as they dont have to steer and they are 9.5 inches wide with a Backspace of 5.5 inches. You need to know this especially if you are lowering a vehicle so with the right Backspacing the wheels will tuck inside the fenders like in this pic and are not to far in to rub on the control arms, leaf springs or inner fenderwells.
Hope that made sense!!