He chose three letters
from among the Elementals
[in the Mystery of the three Mothers
Alef Mem Shin]
And he set them in His great Name
and with them, He sealed six Extremities.
…Nine: He sealed "south" and faced to the right
and sealed it with י ו ה
— Sepher Yetzirah

Yesod is the Astral Light, the Storehouse of Images, the coiled energies of Kundalini, the cycling energies underlying matter. It is Ego, the reactive or emotive portion of I.