As he, himself, explains it, it's a measure of the deviation between a team's actual winning percentage and what one would expect from its game-by-game efficiencies.
As explained by a Reddit contributor, it is essentially flipping a quarter 100 times (a 50/50 proposition) and it landing on heads 60 times. You'd call that a lucky quarter. "Lucky" teams win more games than you'd expect them to based on their statistical profile over a season. In an infinitely long season, every team would have a luck figure of "0" as those factors would balance out over a long period of time.