Wins Above Replacement
Wins a player adds over a freely available replacement.
What it actually is. Wins above replacement: an estimate of how many wins a player was worth compared with a freely available fill-in, with hitting, baserunning, fielding, position and playing time all converted into runs and then into wins.
What a scout reads off it. It is the only number that lets you compare a catcher, a shortstop and a designated hitter on one scale, and that is what it is for. Treat it as a range rather than a point: two players inside a win of each other are tied, and the different published versions disagree with each other by about that much because they measure defense differently.
Where the lines are. 8 and up is an MVP season, 5 is an All-Star, 2 is a solid regular, 0 is what a replacement-level call-up would have given you. Babe Ruth's career total is around 180 depending on whose version you read.
How it lies to you. The defensive component is the noisiest part and needs about three seasons to settle. Trusting one year of defensive value is where most arguments about this number actually start.
Batter's Eye: look up anything in baseball