Wheel Backspacing

[clydethepirate]

Can someone explain wheel backspacing to me.
What I'm wanting to do is run a 31X10.5 tire on a 95 Wrangler with 2.5" Lift.
How do you measure backspacing?

 

[Jeepergeo]

Wheel Offset and Wheel Backspacing – 4 Wheel Parts Stores

The link above explains it.