Win Probability Added
How much each play moved his team's chance of winning.
What it actually is. Win probability added: how much each plate appearance moved the team's chance of winning, added up.
What a scout reads off it. This measures the moment rather than the season, and that is both its charm and its limit. It is the right tool for what happened in a game and the wrong one for how good someone is, because it pays a hitter far more for the same swing in the ninth than in the second.
Batter's Eye: look up anything in baseball