A pianist, no longer young, looks at her or his own aging body and decides to treat it as a gift rather than a loss. The wrinkles are compared to cracks in a once-sleek surface and to ripples on a pond, and the sagging flesh and clawlike hands get an honest look. But the speaker says these same fumbling hands have found a way to go on "without regret."
From there the poem opens outward. Blurred outlines, a faded November sun, and the in-between notes of the piano all become signs of a looser, softer way of living. The image of spent shell casings is the key one: once emptied of their purpose, they have "room for more world." That means more resonance, more give, more play.
The mood is tender, wry, and quietly joyful. The speaker never denies decline, but finds freedom in it. The closing line, "Holding without having, we learn at last how to keep," sums up the thought: loving something without gripping it is how we really hold on.