In this sonnet, the poet compares a single musical note to the young man and a chord made up of many notes to a family. The marriage of sounds in a chord symbolizes the union of father, mother, and child.
The first twelve lines elaborate a comparison between music and the youth, who, should he marry and have a child, would then be the very embodiment of harmony. But music, "the true concord of well-tuned sounds," scolds him because he remains single — a single note, not a chord. By refusing to marry, the youth destroys the harmony that he should make as part of an ensemble, a family. Just as the strings of a lute when struck simultaneously produce one sound, which is actually made up of many sounds, so the family is a unit comprised of single members who function best — and most naturally — when working in tandem with one another.
concord harmony, agreement.