its an advanced metric in baseball that calculates that players worth vs a replacement player. It stands for Wins Above Replacement. And it takes both offensive and defensive stats. The number accumulates each year over year. A good WAR for a season is over 4. Trout avergages over 9 per season I believe. He had his first major injury last year (broken hand) and missed 5 weeks I think, but before the injury he was on pace to break the single season WAR record which I think is held by Babe Ruth.

If you go to Baseball reference you can look at the career WAR for the best players of all time. The players at the top of that list are the top players of all time.