Significance of one divine essence/nature

All human beings share a common nature/essence, yet are not one "being", but the 3 persons of the Trinity share a common essence/nature and are one being. I have 3 questions here: 1. What exactly does "essence" mean? I've seen a lot of answers like "nature" or "being" but that doesn't really explain the meaning of the term. 2. What is the significance of sharing one divine essence/nature? If humans have a common essence/nature but are not in perfect union, why does sharing one divine essence/nature mean the 3 persons of the Trinity are in perfect union? 3. What is it that makes the perfect union (and the biblical-basis for it)?