And those who were seen dancing were thought to be insane by those who could not hear the music.

This quote means that people who are doing something unusual or out of the ordinary can appear to be crazy to those who don’t understand their motivation or the reasoning behind their behavior. Those who dance to an unheard music appear to be out of touch with reality. The quote suggests that those who can’t understand the unseen music can’t appreciate the inner joy and motivation of the people who are dancing.