I copied this image from Baseball-Reference. They have a detailed explanation of WAR (wins above replacement). WAR is better thought of as a framework than a statistic. There are a lot of assumptions behind WAR valuations and since I use those numbers a lot I thought I might talk more about WAR. Here is a graph that is the essence of things:
The first assumption is that baseball talent is normally distributed. It is Gaussian, that is, you get a bell curve when you graph it.