Adjusted ERA+, as seen on Baseball-Reference, is earned run average, adjusted for the ballpark, compared to the league average. It is scaled like an IQ score — over 100 is better than average, 100 is exactly average, and under 100 is below average. A pitcher with an ERA of 4.00, pitching in a ballpark that favors neither hitters nor pitchers, in a league where the average ERA is 4.00, will have an ERA+ of 100. He is the definition of average. If his ERA was better, say 3.50, his ERA+ would be above 100; if his ERA was worse, say 4.50, his ERA+ would be below the average of 100. BUT, if our mystery pitcher had an ERA of 4.00 and was pitching his home games in Coors Field, his ERA+ would be better than 100, to reflect of the difficultly of pitching in Denver. Verse-vicea for someone pitching in Dodgers Stadium.
ERA+ is particularly useful for comparing pitchers across time periods. For example, in 1968, when run scoring was dramatically low, the National League average ERA was 2.99. That season, Tom Seaver posted a 2.20 ERA, good for an ERA+ of 137. In 2008, Johan Santana pitched to a 2.53 ERA in a league where the average ERA was 4.29. This gave Santana an ERA+ of 166. Tom Seaver’s ERA was better than Johan Santana’s (2.20 to 2.53), but Santana’s adjusted ERA+ was better (166 to 137) because he pitched in a league that scored 43% more runs (4.29 to 2.99). This meant that, relative to each one’s league, Johan Santana was better at preventing runs than Tom Seaver was forty years earlier. ERA+ is built to reflect things like that.
These are the Mets all time ERA+ leaders, minimum of 500 innings pitched:
Santana has a lead on Seaver for the moment, but Santana’s ERA+ will presumably drop as his performance declines with age. By the time Santana’s Mets career is done, Seaver will probably have regained his position on top of this list. But until then, there is an argument to be made that, inning for inning, Johan Santana is the most effective pitcher to put on a Mets uniform.