A foundation is a flat, rigid surface made popular in the last 10 years by companies using dense foams, like memory foam and latex, in their mattresses. When dense material is used in the mattress, you can put the mattress on concrete and how it feels is somewhat independent of the surface it’s resting on; it still feels comfortable. Compared to box springs, which are flexible, foundations are rigid.  A foundation is the best choice for a mattress made with natural rubber (natural latex).

How does it differ from a box spring?

A box spring is a rectangular “box” made with springs inside it that flex along with the mattress. The springs may be coils or thicker pieces of wire, but the idea is that it moves up and down as the mattress moves. 100 years ago they were made so that the manufacturer could make a firm mattress, but have it feel a little softer because the box spring flexed. It also allowed them to offer a longer warranty.  A box spring is a good choice to pair with a natural innerspring mattress.