A foundation is the base for something. You often hear about the structural foundations of buildings, but a good idea can also be the foundation of a successful business.
In the simplest sense, foundations support something else. Friendship is the foundation of a good marriage, and many svelte figures rely on girdles (known to underwear specialists as a foundation garment). But foundations are also charitable organizations. And in cosmetics, foundation is gooey make-up that's supposed to match your skin tone.