How to interpret the win / draw / lose values

When starting a new game, the following is displayed after listing the opponent, as an example: win +4 / draw -4 / lose -12

What does this mean and how to interpret this?

Thank you

Thank you!  Great explanations!

These points changes from game to game, depending on the ratings of the players.

The points gained or lost also change depending on how well established the players' ratings are. A 1600-rated player who has played only six games in the past three months will experience a larger point-swing than a player with an identical (1600) rating that is based on dozens of recent games.

That's because this site uses Glicko ratings, which incorporate a "hidden variable" that tracks not your rating but the statistical signifigance of your rating.