Dave Bonta's poem uses the fretless banjo as a way of talking about freedom from fixed guides. A fretless instrument has no metal bars to tell your fingers where the notes are, so you have to find them by feel. The speaker compares this to walking in the dark, and says maps and even vowels can't be trusted. Rules and markers, the poem suggests, don't always help.
The speaker seems relaxed, even delighted, about this. Images pile up in a playful, slightly odd way: goats living on notes nobody else sings, a glass ball on a lightning rod that longs to break free, a few katydids carrying a whole night's sound. Each one points to something small, improvised, and enough on its own. The biblical touches, Canaan and Jubilee, give the mood a sense of settled, everyday celebration. The closing question, "Who are we / that we should fret / over bars of brass?", is a pun on "fret" that sums it up: why worry about rules when you can play by ear?