Binah, Understanding, is the third sefirah, the supernal mother, Aima, who receives the undifferentiated creative force of Chokmah and gives it form, boundary and structure, the womb in which the seed of wisdom becomes something that can be born into the created world. She heads the left-hand Pillar of Severity and completes the three supernal sefirot above the abyss.
Binah is understanding as distinct from wisdom: where Chokmah is the flash, Binah is the comprehension that shapes and contains it, and she is therefore associated with form, limitation, and the sorrow that form imposes on the boundless, the Kabbalists linked her to the great sea and to grief. Her divine name is YHVH Elohim, the fullest of the divine names, joining the ineffable Tetragrammaton with Elohim. The doctrine of the sefirot in which Binah takes this place is the achievement of Jewish Kabbalah, named in the Sefer Yetzirah and unfolded in the Zohar, where the interplay of the supernal father and mother is a central theme of the divine inner life.
Sources The ZoharTraditionally attributed to Moses de Leon with Sefer Yetzirah: The Book of Creation in Theory and PracticeAryeh Kaplan