From what I understand, he means that if you bluff too much people will start challenging your bluffs so they're not as profitable. And if you bluff too little, they'll see that you usually bet for value and won't pay you off as much when you have a legitimate hand.
So ideally, you want to bluff that unknown
percentage of the time that matches their pot odds
. Hence they can only make mistakes, either calling too much or folding too much. But as I stated, that percentage can only be guessed at because there are too many unknowns.