If a player is + in plus – then the team has outscored the opponent while that player is on the floor. If your +/- is good then your team is outscoring opponents while you are on the floor so that is why it is a good thing.
It is a stat that was initially used in hockey for goal differential but has been extended for use in basketball with point differential. It has significance as an advanced metric because it can reveal value that is not immediately noticeable in a traditional box score (points, assists, steals, blocks, etc.). The Houston Rockets used a version of this stat which helped reveal the value of the low scoring Shane Battier.