when writing a program for a specific purpose, the whole problem usually has to be fragmented into elementary/basic tasks in order to become manageable. If I follow well the ST, what the author is saying is that by asking yourself the right questions at the conception stage, you can find the best way to cut the whole problem in basic "building blocks" i.e. elementary tasks, each one corresponding in the next step to a segment (often re-usable) of programming code.
découpage" in the ST is about deciding what would be the "building block"/segments of the programme about to be written, what will be its general structure.