Chesed, lovingkindness, is God’s overflowing generosity, associated with Abraham; Gevurah, also called Din, judgement, is the power of limit and restraint, associated with Isaac. The world needs both in balance, harmonized by Tiferet. When judgement is unbalanced, the kabbalists said, evil arises.
Chesed and Gevurah
Lovingkindness and judgement: the fourth and fifth sefirot, divine expansion and restraint.
Also: Hesed, Din