It is a sturdy and flat support structure typically constructed of wood and often used in conjunction with a wooden or metal bed frame to elevate a mattress off the ground and provide a firm surface that will not sag over time under the weight of the mattress and the people on it. A quality mattress foundation/base extends the life of a mattress and it is designed to:

• Act to provide an even rigid and non-flexing surface to support the mattress and make sure that any compression is all in the mattress itself. This would apply mostly to foam core mattresses (without any springs) which can absorb shock all by themselves without the risk of deformation of an innerspring. They are usually "designed" to be self-contained and to achieve the design goals they require an evenly supportive non-flexing surface.
• Act to provide ventilation under the mattress. While a solid surface platform or foundation would be the most supportive type of support system ... the lack of ventilation under the mattress in combination with other risk factors can contribute to the risk of mold or mildew or the accumulation of dust mites in a mattress (see post #10 here ).
• Some Foundations have attachable legs of various heights to raise them off the floor, but they are typically placed on a wooden or metal bed frame. It comes in various height options that allow the surface of the mattress to be at a certain height.

Caution: Before purchasing a mattress foundation/base it is important to always check with the mattress manufacturer to ensure that the support system is suitable for your mattress and doesn’t void its warranty.