Resonance occurs between things that are, in some way or another, attuned. You pluck a string on one violin, the string of another, tuned similarly, will vibrate. A swing pushed in keeping with its natural frequency goes higher and higher; trying to push against that natural frequency dampens the motion, and makes the arcs smaller.
What is resonance in art? Think of resonance in everyday life. Usually it's an all-to-brief moment when the quotidian noise in which we embed ourselves lifts like the fog. For that moment the noise is silenced, one feels attuned, aware, present, and alive. Depending on your own make up, just about anything can be a trigger. Maybe it's a kitty picture on Facebook, maybe it's sitting on hill watching the sun rise. Maybe it's a poem, or a bit of music. Your spirit vibrates, all-too-briefly, in tune with something greater - or at least other - than yourself.
In art, that in-tuneness, if practiced, sets the creative imagination free. The resonance between external and internal perception, if allowed to develop, drives an interaction between the conscious and unconscious mind which results in mental metaphors for the experience. For those of us who are visually oriented, those metaphors are experienced as potential images. The art results as the artist reifies the potential into the real and external, which again becomes part of the experiencing.
As for freedom, in physics, a body can have as many resonant frequencies as it has degrees of freedom. A simple pendulum has one, a coupled pendulum has two, etc. How many degrees of freedom are there to the human spirit? Who knows, they seem pretty endless. But we'll never know without exploring further.