The principle is that God's Essence (ousia) is distinct from His Energies (energeiai) or activities in the world, and it is the energies that enable us to experience something of the Divine. Orthodox theology holds that while we can never know God as He is in Himself ("Essence") and that direct experience of God would simply obliterate us (much as Moses could not survive seeing God's face), God's "Energies" can be directly experienced (as Moses could see God's back and live).
The doctrine was given full exposition in the 14th century by St. Gregory Palamas, who defended the practice of Hesychasm against accusations by Barlaam of Calabria that to claim to experience the energies of God was tantamount to claiming equality with God. In defense of his teaching, Gregory Palamas quoted St. Basil of Caesarea: "It is by his energies that we know our God; we do not assert that we come near to the essence itself, for his energies descend to us but the essence remains unapproachable."