To put is as briefly and simply as is possible for me, the idea is that God’s very essence is “to be” eternally, and to be the source of His own being.
To better understand this it’s helpful for us to look at how we (and everything else) are different. The essence of Ghosty is “maybe to be”, not in the sense that I only maybe exist, but in the sense that sometimes I’m here, and sometimes I’m not. There was a time when I didn’t exist, though there will never be a time when I no longer exist, so my essence doesn’t have the trait of existing absolutely.
What’s more, God’s essence IS His being which is also different from us. I “am”, but not absolutely due to my essence. The being, or existence, of my essence finds its source outside of me; my parents, the Earth, God, a whole chain of events and causes. You can’t look at me and say “the reason Ghosty exists is because he’s Ghosty,”; my existing is distinct from my essence.
With God, on the other hand, His being and essence aren’t separate at all. His essence is “to be”, and He is nothing other than His essence (He’s not made up of extraneous traits or causes like you and me). Since He is nothing but His essence, He doesn’t change or fluctuate, and this also plays into Him being His own being. If God changed, He couldn’t be the source of His own being because there wouldn’t be the “same” God from moment to moment, and His existence would be founded on something that wasn’t there. Since out being comes from outside ourselves, in God, we can change without losing ourselves precisely because God DOESN’T change and fluctuate.
So to briefly answer your questions, it’s important to metaphysics because we know philosophically that at the root of all “somethingness” there must be an absolute being who doesn’t change, and is its own essence and being. This is knowledge that can be arrived at without Divine Revelation, and we call this “entity” God; combined with Divine Revelation we can actually come to know this entity personally, but the point is simply that you don’t even need Faith to know this much.
God’s essence is His being, and visa versa. He is what He is because that’s what He is, and that’s all that He is.
His essence is SO different from ours in how it “functions” that it’s actually beyond our comprehension. Human understanding unaided by Grace can only make analogies based on how God must be different from us and all creatures, but it doesn’t give us much insight into what God is like. In other words, it’s like a blind person saying “that door is green”; it’s true, but they have no concept of what they are describing.
The essence of being a human being is too great to quantify here, and is possibly also incomprehensible to the human mind. There are certain features that stand out that are relevant to this topic, however, such as our rationality and our contingent existence.
Does that make things clear as mud for you?
BTW, I STRONGLY recommend “Theology and Sanity” by Frank Sheed for a strong and accessible layman’s introduction to these concepts.
Peace and God bless!
That made complete sense