Learn in the textbookCoding Conventions and Structuring
DefinitionModule decomposition (モジュール分割) is a technique of splitting a large program into parts by function, called modules, and assembling it from them.
ExplanationEach module is given a single role, and modules are combined after deciding what information they pass to one another. When a change is needed, its effect tends to stay inside that module, and parts already built can be reused in other programs. The word module is also used for the program units produced when source code is translated and linked.
AnalogyIt is like furniture built from boards, legs and drawers, where a broken drawer can be replaced on its own.
Memory hookModule means a standardized, interchangeable unit, like a spacecraft's habitation module, a part that can be swapped or added.