What WAR measures
Wins Above Replacement (WAR) is a summary statistic that estimates how many more wins a player contributes than a freely available replacement-level player, such as a minor league call-up or waiver pickup, would in the same playing time. It rolls offense, defense, baserunning, and (for pitchers) run prevention into a single number expressed in wins.
The replacement baseline is the key concept: WAR is measured against a low, readily replaceable level of production rather than against an average player, so even an average regular accumulates positive WAR.
How it is built and its limits
Position players' WAR typically combines batting runs, baserunning, fielding, and a positional adjustment, converted to runs and then to wins using the rule of thumb that about ten runs equals one win. Pitchers' versions use either runs allowed or component metrics like strikeouts and walks.
There is no single official WAR: Baseball-Reference (bWAR) and FanGraphs (fWAR) use different defensive and pitching inputs, so values differ for the same player. WAR is best treated as an approximate, big-picture estimate rather than a precise figure, especially because defensive components carry the most uncertainty.