Mafioso slang for a mob employee who is a hard worker and of good value to the family. Typically someone is considered a "good earner" because they bring in more money to the organization than they take out. An employee could also be considered a good earner if he/she exhibits a special skill set, such as a keen ability to remember numbers, or being very good at killing people.
People inside the Gambino family wanted to whack John Gotti early in his career because of some outlandish stuff he pulled, but they kept him around because he was a good earner.
Paulie? Why you fuckin wanna whack Paulie? Paulie's a good earner. You're not fuckin whackin Paulie.