A crumb is a very tiny piece of food. Some recipes instruct you to top a dish with bread crumbs before you bake it.
Crumbs are what you're left with after finishing a box of cookies or a bag of tortilla chips — the bits that are too small to eat. You can also talk about other, non-edible types of crumbs, like the crumb of wisdom in an otherwise silly movie or the crumb of information a detective finds at a crime scene. The Old English root is cruma, "crumb," and it's thought that the silent b was influenced by words like dumb.