What does Beatty tell Montag happens t every fireman sooner or later?

From the text:

Beatty puffed his pipe. "Every fireman, sooner or later, hits this. They only need understanding, to know how the wheels run. Need to know the history of our profession. They don't feed it to rookies like they used to. Damn shame." Puff. "Only fire chiefs remember it now." Puff. "I'll let you in on it."