WAR
Wins above replacement — total value (offense + defense + baserunning) vs a freely-available bench player.
What WAR means
Catch-all valuation: a 5-WAR season is All-Star caliber, 8+ is MVP. Different sites have different formulas (fWAR vs bWAR) but the scale is broadly the same.
What counts as a good WAR
Higher is better — roughly 0 is poor, 2 average, 4 good, and 6 or better elite.
Also written
war_value · fwar · bwar
Source
FanGraphs / Baseball-Reference