Just as a building needs strong foundations if it is not to fall down with the first strong wind that strikes it, each asana (yoga posture) needs a strong foundation upon which to build its other components- stability, alignment and extension. In order to maintain the posture and extend out from it, there must be a strong base.

Whilst in some poses such as adho mukha svanasana (downward facing dog), the hands are part of the foundation, in many poses it is the feet which form the foundation. 

If you build a firm and level foundation by focusing on your feet, from it a strong, spacious, and elegant pose will rise. To create a firm foundation, use the strength of the legs to send strong roots into the earth; make the foundation level by contacting the floor evenly with the feet. Building awareness in the feet in each pose is extremely important, as if the feet are subjected to incorrect alignment you will eventually end up with strains or more serious injuries.

The same applies to the hands where they form the foundation of the pose.  The pull of gravity is what draws us towards the earth. Gravity is like a magnet attracting us to the earth, but this attraction is not limited to pulling us down. It also allows us to stretch in the opposite direction towards the sky. The pull of gravity under our feet makes it possible for us to extend the upper part of the spine, and this extension in turn allows us to release between the vertebrae. Thus the stability and extension of the pose, as well as the capacity correctly to align the pose are all dependent upon the foundation.