The speaker addresses a musical saw, imagining its life as a tool and its new life as an instrument. Once it cut straight lines through wood, doing the "harsh" work of service. Now it is held still and played with a bow, a delicate thing far removed from the arrow-like purpose of a blade. The speaker celebrates this change: the saw is "free," and its song might never have existed otherwise.
The tone is playful and tender. The speaker pictures the saw as a body putting itself into the music, and even gives it a heart of sorts, the "sweet spot" street musicians search for. But the joke is gently deflated: the saw has only "cartilage," not a real heart. The ending turns the mood. The clear, high note rises in what sounds like joy, yet the final question, "Or is this grief?", leaves the celebration uncertain. Perhaps the saw's beauty comes with loss.