Replacement Level
What a team gets from a freely available fill-in player. WAR measures from here.
How it actually works. It is the zero point that wins above replacement is measured from, set so that a whole team of such players would win roughly 48 games in a 162-game season.
Why it is done, and when it is wrong. It matters because average is the wrong baseline. An average player is valuable and there are not enough of them; the real alternative to signing somebody is the man already in Triple-A, and that is the comparison a front office actually faces.
Batter's Eye: look up anything in baseball