Suppose you want to build a bridge over a canyon. You throw a ball tied to a string across the canyon. Then use that string to haul over heavier and heavier cords. In the same way, you identify your theme/main plot line or single defining sentence of your character’s journey. Then, when that’s set, you strengthen it with character, setting, and so forth, building the structure of your novel.
Seems reasonable to me. Writers are often divided into “pantsers” (writing by the seat of the pants, starting a novel without knowing where it ends) and “plotters” who start with the basic plot and build and build on that. I’m a plotter for sure. The canyon is huge and I don’t want to just leap and not know where I’m ending up.
For all three of my novels, I’ve had an image of the first and last scene in mind: who is there, what they’re doing, the mood, the voice. And I’ve got a picture of the first scene. Then to connect them. It’s still scary, but at least I know where the bridge is.