Binah is the first feminine Sephirah. It is the female that we encounter the concept of nurturing. Binah is impregnated by the "thought" of Chokmah. This thought is nurtured to produce something that is greater than the sum of its parts (Chokmah + Binah). The number associated with Binah is 3. Here the point of Kether, manifested in the line of Chokmah, becomes fixed in the triangle of Binah.
This Sephirah holds within itself the primal concept of form. Without Binah the physical forms manifest in the created universe would not exist. Binah gives the basis of forms that are actualised by the force of Chokmah.
Binah means "understanding". In the human mind understanding is exhibited when wisdom (Chokmah) and knowledge are applied. It is Binah that sets the structured framework for this application to take place.
Binah is associated with Saturn.