+/- is the difference of points scored between you and your opponent while a particular player is on the court.
For example, if player A has a +/- of +7, this means "while player A was on the court, your team scored 7 more points than your opponent's team"
If player B has a +/- of -23, this means "while player B was on the court, your team scored 23 points less than your opponent's team"
If a player plays only a couple minutes where neither team scores or both teams score the same amount, the +/- is 0.
If a player plays the whole game without subbing out, the +/- would equal the points differential of that particular game.