Alternately, it implies that part of the DM's job is to learn to prep content generators - things that make it easy to determine things on the fly - rather than content whenever possible. That could be everything from prepping things in a modular manner so that you can reuse elements of your prep, to just mapping out motivations and goals and general themes and then filling in the blanks only when they're needed, to doing everything full on improvisation all the time.