The term "crumb" can have several meanings depending on its context.
In a general sense, it refers to small pieces or fragments of something. For example, when talking about bread, crumbs are the small bits that remain after you've taken out large chunks. In this usage, "crumb" is simply another word for "fragment."
However, in some specific contexts, "crumb" can have a more particular meaning. For instance, if you're talking about the crumb of a pie or cake, it refers to the small pieces of dough or batter that remain after the main part has been baked and removed from the pan. In this usage, "crumb" specifically describes the texture or consistency of the leftover bits in the bottom of the baking dish.
Overall, whether you're talking about fragments or textural residue, the term "crumb" generally refers to small pieces or remnants of something.