Learn in the textbookCoding Conventions and Structuring
DefinitionNesting depth (ネストの深さ) is how many levels of nesting are stacked, where a conditional branch or loop is placed inside another conditional branch or loop.
ExplanationFor example, a loop containing a branch that contains another branch is nested three levels deep. The deeper the nesting, the more levels you must trace back to see when a line runs, which invites misreadings and faulty fixes. Deep parts are kept shallow by measures such as moving them out into separate processing.
AnalogyThink of boxes inside boxes inside boxes: the innermost box takes the most opening to reach.
Memory hookNest means a bird's nest in English: just as eggs sit in a nest, processing sits inside processing.