What is the Best earned run average in baseball?

ERA is the average number of earned runs a pitcher allows per nine innings pitched.

If it is a pitching stat then it is the average runs a pitcher allows in nine innings. If it is a hitting stat it is an average of how many runs a team scores per game.

if the run was reached or scored because of an error, then it is unearned, any other instance, it is earned

Earned Run. Earned runs allowed (all runs given up not due to a fielding error) multiplied by 9 , then divided by innings pitched.

It is a measure to judge how effective a pitcher is. If it calculated by taking the total earned runs a pitcher has allowed and dividing by (total #of innings pitched/9). Giving you an average number...