If something is crumbly, it breaks easily into small, dry pieces when you touch it or bite it. Think of old cheese, a biscuit that falls apart in your hand, or dry earth in the garden. The pieces are like crumbs, not one clean snap. People use it a lot for food, and also for stone, walls, or soil that is not solid. It often suggests the thing is a little dry, old, or loosely put together. That can be nice — a good crumble topping is meant to be like this — or a problem, if you wanted the biscuit to stay in one piece. The word is ordinary and a bit homely, not a science term.