Yesod, Foundation, is the ninth sefirah, positioned on the central Pillar of Equilibrium directly below Tiphareth and directly above Malkuth, and it is the channel through which all the energies of the sefirot above are gathered and transmitted into the material world of the tenth sphere. It is the foundation in the sense of the base on which the created world rests, the reservoir and conduit of the divine influences, and it is traditionally associated with the Moon, with the astral or etheric level, with imagination, dream and the generative and sexual powers.
Yesod collects the flow of the whole Tree and focuses it downward, so that nothing reaches Malkuth except through it. Its divine name is Shaddai El Chai, the Almighty Living God. In the Kabbalistic scheme Yesod is the vital foundation and the sphere of the tzaddik, the righteous one who channels blessing into the world; it takes its place in the doctrine of the ten emanations named in the Sefer Yetzirah and developed through the Zohar, where its mediating and generative role between the divine and the created is a recurring theme.
Sources The ZoharTraditionally attributed to Moses de Leon with Sefer Yetzirah: The Book of Creation in Theory and PracticeAryeh Kaplan