"Resonant" means that your elnin's mana flows smoothly and is in harmony with the greater flows of energy through Eyre. The resonant status is currently only used to reduce the rank cost of growths and/or rank upgrades, but could evolve further uses as we are able to expand our ARPG.
Lore wise, an at rest resonant elnin gives off a pleasant "thrumming" vibe when perceived by other mana-sensitive creatures. Nothing overt or distracting, more like a silent, subtle purring- the kind of thing that you tune out immediately, but gives you a sense of calm and relaxation on a subconscious level. The company of resonant elnins has also been proven to lower your blood pressure and reduce stress.