"stuffin the tires" is what happens when you stuff a tire up into the wheel well, causing rubbing. The stuffing is caused when sufficent lift is lacking to prevent the wheel from hitting the body. In other words, when your out wheeling, and you choose to drive up this knarley ravine you off to your left, and your truck happens to stay somewhat level, but your running gear is "Crossed up" major, chances are, if you don't have a lifted suspension, some of your wheels will be "stuffed"
Which is what we all wheel for anyhow. (one of the reasons anyway!)
Backspacing. Laymen's terms, basically, the center of the rim, (where it bolts to the vehicle), it's position within the rim as measured from the inside edge, to the the center. Lay a board across the rim, then drop a tape measure down to the center. Thats the "backspacing" backspacing is important, in that some tires with rims with incorrect backspacing, can cause the tires to hit the vehicle when turning.
There are others out here that can probably explain it better, but that's how I look at it.
Troy