The +/- stat is calculated this way:
You get a +1 for every point your team scores while you are on the court, and a -1 for every point your opponent scores when you are on the court. Your +/- stat is the compilation of them.
So, for instance, if while you are on the court, your team makes a three-pointer, two dunks and a free throw (+3, +2, +2, +1 = +8), but the opponent makes two layups and two free throws (-2, -2, -1, - 1 = -6), your +/- for the game will be +2 (+8 -6).