The foundation is usually in cream form and when applied and blended well, provides a uniform tone & texture to the skin for further application of makeup products like compact, blush, highlighter, etc. You can buy foundation online specific to the shade of your skin so that it does not look strange compared to the skin on the rest of your body.
Make sure you always buy a foundation that suits your skin tone perfectly. Your foundation is meant to give your skin a beautiful even tone finish and should not look weird. Also, if you pick a foundation with a tone lighter than your own skin tone then your foundation will appear to look like it has oxidized. And a tone darker would look unnatural.
The best makeup combo usually has a few variation options to choose from & will have a foundation to act as a base for your color cosmetics. Foundation will help bring out the maximum finish, color & brightness from your makeup.