It is often associated with the Moon, because it is the sphere which reflects the light of all other sehphiroth into Malkuth, and it is associated with the sexual organs, because it is here that the higher spheres connect the earth.
It plays the role of collecting and balancing the different and opposing energies of Hod and Netzach, and also from Tiphareth, above it, storing and distributing it throughout the world. It is likened to the 'engine-room' of creation.
The archangel of this sphere is Gabriel, and the Cherubim is the Angelic Order."
Yesod is also associated in the soul with the power to contact, connect and communicate with outer reality-meaning Malkuth in this case. The foundation (Yesod) of a building is its "grounding" its union with the earth (Malkuth).