Earned Run Average details the average number of earned runs a pitcher gives up per nine innings pitched. Unearned runs such as those created due to defensive errors aren't used in the ERA calculation.
ERA = 9 x (Earned Runs / Innings Pitched)
More about Earned Run Average Leaders
Earned runs average leaders are the pitchers who give up the fewest number of runs on average over nine innings that are considered “earned”. The simple formula is to take the number of earned runs allowed over the season, divide that number by the number of innings pitched, and multiply that number by nine. Whether or not a run allowed is “earned” can be something of a judgment call by the official scorekeeper; a run can be considered unearned if the opposing player who scored reached base or advanced a base due to an error or passed ball (though advancing an extra base on an error is not a guarantee that a run scored by that player will be considered unearned). Earned run average is a very good statistic to measure a pitcher’s effectiveness, though some newer advanced statistics can be even more precise.