String theory is a framework that describes the nature of sub-atomic particles as one-dimensional objects called strings. The vibrational state of these strings is what defines mass and other properties of the fundamental building blocks of the universe.
The connection between resonance and substance can also be witnessed in Resonant-Transmuters. This class of artefact can change their substance and form based on the frequency of oscillation. Various interactions with Resonant-Transmuter can introduce a new oscillation and immediate transformation. You may have experienced this effect, say, while playing the radio too loud at home, only to discover your dinner table has changed shape suddenly.