After Stevens calls the modern poet (or poem) a "metaphysician in the dark," it starts to sound like he's getting too grandiose about his role as a poet. So to take some of the starch out of his shirt, he says that his metaphysician is like someone just plucking a single string on a guitar. Modern poetry might not be the most sophisticated stuff. After all, Stevens is writing in total free verse, which would've seemed crude and barbaric to poets in the past.
In lines 20-21, the modern poet or poem is like a person "Twanging/ An instrument" in the dark, sending the note out into the darkness like a ray of hope. The note isn't anything complex. It's just a tiny symbol of hope that modern poetry keeps sending out to comfort us in dark times.
The important thing for Stevens is the hope that's symbolized by a single guitar note flying off into the darkness. Poetry will always be a pretty blunt instrument for capturing the complexity of the world. But the important thing is that poets keep striving to find something meaningful in human life.