The speaker starts out cheerfully admitting that he is proud of his own signature. He describes it in grand terms: sweeping strokes, a V like the prow of a racing snake boat, a slant that makes it look like a Roman galleon about to ram an enemy ship. It is a funny picture, since this mighty vessel mostly turns up on scraps of paper, office vouchers and letters to the editor.
Then the tone shifts. He notices that the signature changes with his moods, rising and falling "in the tides of my mind." He also admits it is really shouting "look at me," a vain attempt to impress. The last lines are quietly rueful: the signature has hardly changed in nearly forty years, and neither has he. The poem begins in boastful play and ends in gentle self-mockery, with a small, honest recognition of how little a person may change.