Yesod is the ninth of the ten sefiros, and the sixth of the emotive attributes within creation. It appears in the configuration of the sefiros along the middle axis, directly beneath tiferes.
Yesod represents the power to contact, connect and communicate with outer reality. For this reason, the sefirah of yesod is identified in the Torah with the tzaddik as it says: “and the tzaddik is the yesod (foundation) of the world.” In particular, this refers to the one, perfect tzaddik of the generation, or to Joseph (son of Jacob). In the very body of the tzaddik, which is finite and limited in time and space, God’s infinite light and creative life-force becomes manifest. The tzaddik procreates on the spiritual plane by connecting the people to each other. Joseph interacted with the whole world, when he supported them, in the years of the ancient Egyptian drought.
Yesod is also referred to as the bris, the holy sign of the covenant that God made with Abraham. It’s known as the ‘the holiness of Israel’. It corresponds in the human body to the procreative organ and it’s something that needs to be protected greatly. We see this again with Joseph’s endurance in rejecting illicit relations with his master Potiphar’s wife.
Rebbe Nachman, the perfect tzaddik of the generation before Moshiach, taught that when a man damages the attribute of yesod with a willful emission of seed, God forbid, he creates klippot that cause him to be depressed and despair. The Rebbe revealed a remedy for this pollution by reciting the Tikkun Haklali, a specific ten chapters from psalms.