A box spring is not something that you should ignore if you are considering upgrading your bed or buying a new one. The first thing to note about box springs is that they are misnamed. Many box springs do not actually contain springs, though some do. With or without springs, a box spring's role is mainly one of support, in that it is the base on which your mattress sits. That answers the question of what it is, but not the one of why you need it.