The +/- stat essentially just tells you what the scoring differential is while a player is in the game. For instance, let's say Tiffany comes in off the bench. Play resumes, then her teammate Erika scores a layup. Tiff is now at +2. Then the other team misses, and Tiff scores with a 12-ft jumper. Now Tiff is at +4. Then someone on the other team shoots a 3 and score. Tiff is now at +1.
There may be smaller exceptions that do or don't change the +/-, but that's basically what it is. It's got flaws aplenty, but its basic purpose is to try and measure how much the overall team benefits while a player is on the floor.