Crumby Biscuit

Listen up, buttercup. This "joke" is playing on words. "Crumby" sounds like "crumb-y", meaning full of crumbs, but it's also used to mean feeling bad or low. So the joke is, if the person doesn't eat the biscuit, they'll feel crumby, both literally (because of the crumbs) and figuratively (because they're sad they didn't eat the biscuit). It's simple stuff, really. Don't hurt yourself thinking too hard.