It's a measurement, for each player, of their team's total scoring vs the opponent's team when that player is on the floor. The purpose is to provide another gauge of a player's impact on the game. In other words, it is an effort to measure a the fact that many players impact a game in ways other than traditional measurements (i.e. defensive rotations or ball movement, etc as opposed to scoring, rebounding. or assists).
The perceived flaw is that it uses a total team metric to analyze an individual player's impact, but it's still an interesting indicator.
There are many Baylor fans who believe that Lauren's impact on a game goes far beyond traditional measurements such as scoring, rebounding, or assists. That she does so many little things with her defense and passing which don't get measured (deterring drives to the hoop with her shot blocking presence, subtle defensive rotations and positioning, making the pass which leads to the assist, making the screen which opens the driving lane, offensive spacing which creates better angles, etc). Lauren should be the Queen of the +/- rating.
If you look at the Fever stats in your link from the Sky game, every Fever player has a negative rating, because the team was blown out by 23 points. Lauren had the 3rd best +/- on the team with -9, so that's a small silver lining suggesting that she may be having a positive impact on the game compared to her teammates.