Resonance, as the Fallen understand it, are the remnants of the multiple levels of Creation that the Creator originally envisioned. In Paradise, each object existed in several states of being simultaneously, so a flame was also a mathematical equation, a song, a philosophical concept and a plant, all at the same time. This multi-existence was shattered during the Age of Wrath, when God, in His anger, touched Creation and accidentally broke it apart. Still, these connections linger as a form of sympathetic Resonance that connect things to concepts.
Angels and Demons alike used this Resonance in the War of Wrath to create their Lores and Evocations, to make up for their diminished control over creation after the Fall. Many returned Fallen have discovered that Resonance has widened thanks to human invention and symbolism and now, each Resonance encompasses many more concepts and objects than before. Using Resonance makes using a Lore more efficient and focused, although the Resonance has to fall under the purview of the used Lore.