I've read it in a few Mick Foley books. I am lead to believe it is someone who works hard on all aspects of his career. So like, in-ring abilities, mic skills, fan attraction and public image.
So, a good one example is Cena. Early on, he was already athletic, when they gave him the mic, he performed well, with his raps and such, then the WWE helped put him over with the fans, and he has behaved outside of the ring by not using illicit drugs, or causing heat by saying things he isn't supposed to.