Let’s take a break from literature for a bit.
The essence-energies distinction is a way of conceptualizing the Godhead that developed in the east. Medieval theologian Gregory Palamas was the major figure behind crystallizing it. Exactly how far back it goes before him is still a bit out of my grasp, and I think there’s a degree of controversy over whether or not it gets read back into Church Fathers like the Cappadocians.
Any form of classical theism has two realities that need to be related to each other somehow: on the one hand, the absolute transcendence and otherness of God, the immense gulf between God and created things. And, on the other hand, the profound immanence of God in all of creation, constantly sustaining it and operating within it.
As the name implies, the essence-energies distinction approaches this by making, um, a distinction between the essence of God – which is unknowable and inaccessible, and the uncreated energies of God, which permeate the world, and which we can encounter. Love, for instance, would be a divine energy.
This is distinct (har har) from the usual western conception, which denies any such distinction: there is the essence of God, about which we can analogically predicate many attributes (i.e., love, justice) which, while they are conceptually distinct for us are actually one unified reality as the Godhead.* To take the Thomistic formula, God is nothing but the sheer act of existence itself. All individual existences are dependent on God as the ground of their being.
I’m not sure if I grasp the eastern view well enough to say if it ultimately makes sense. The philosopher in me wants to quibble with the idea of there being composition in God, but I’m not sure if that’s what is being conveyed here. Palamas, for instance, is writing in the language of mysticism rather than technical philosophy/theology, and there’s a danger in trying to treat him as if he’s trying to do the latter.
*It is important to note that we are not speaking of the three Persons/Hypostases who share, or rather constitute, that essence. How that gets cashed out is a separate problem in its own right.