It ain’t a make of French car, if that was your answer. It’s a fruit, the candied rind of which is frequently called for in Christmas fruitcake recipes. It’s a head-scratcher for many people, since outside of fruitcake-making, few people ever encounter it. But the reality is that outside of fruitcake-making, there are very few uses for the citron. It’s a citrus fruit (as I’m sure you can guess), though calling it a “fruit” is rather generous, since there’s really no fruit in it. It’s almost entirely pith. Yet the skin is rich in essential oils, and has been prized for its fragrance since antiquity. Brined and boiled, it’s awfully darn good for candying. As for what becomes of the rest of the thing once the rind is taken off, I’ll be darned if I know.