WAR is Wins Above Replacement. Pretty much compared to the average player available at that position in terms of wins. Available could be a minor leaguer or an easily attained free agent.
@wingtee Following on from our professor friend, it is an attempt (and I stress attempt) to account for fielding impact and hitting impact compared to the MLB average. So Alonso has a very good offensive WAR as he is very productive compared to the average MLB hitter BUT his defensive WAR isn’t special. He is essentially average. Part of that is playing a relatively unimportant position and part of it is not being particularly special at that position.
The problem I have is relating a walk to a certain amount of runs and relating defensive coverage and fielding percentage to runs. For instance, a 2 out walk is not as valuable as a 0 out walk. The odds of a no out walk scoring must be higher than a 2 out walk. Same for a great diving catch. Or maybe a better thought defensively is outfield assists. Gunning someone at home is an obvious correlation to a run but throwing a guy out stretching a single to a double with 2 outs? Harder to quantify. Long story short, it has holes from my reading of it BUT it is a more single complete metric for value than any one other metric hence my use.