+20 means that throughout the duration of a game, when a player was on the court, his/her team outscored the opponent by 20 points.

This statistic is usually referred to as plus/minus.

Suppose a team wins a game by 4 points.

Player A scored 32 points and finished with a plus/minus of -7.

Player B scored 5 points and finished with a plus/minus of +16.

Even though player B didn’t score many points, she may have made contributions in ways that are not represented by conventional statistics. Like playing shut-down defense, for example.