A very old musician plays a hammered dulcimer, the instrument that came before the piano, and plays it with real skill. The poem watches him work and notes that he knows exactly which vibrations a steel string will give when it bends and springs back. Then it turns to a question: are those vibrations really so different from the blood moving from heart to hand?
That question is the heart of the piece. The speaker seems to see the musician and the instrument as two versions of the same thing, each a system of tension and pulse. The hand strikes the string, but the hand is itself driven by a rhythm inside the body. The title, "Semaphore," fits this idea of signals passing along a line, whether through wire or vein.
The mood is quiet and curious, almost reverent. There is admiration for the old player's long practice, and a gentle wonder in the way the poem ends, not with an answer but with an open question.