The speaker has just earned a driving licence at forty-eight, and he's a little self-conscious about it. He stands among teenagers, grey-haired and balding, and describes the test, where he steers a car through an "H" shape between iron rods, like a baby taking first steps. Gradually the poem turns this small achievement into a picture of his whole life.
The car's controls become metaphors. The clutch, the throttle, the brakes and the gears stand for how he has paced himself over the years. He admits he has always drifted to the left, "just a bit to port," following his own conscience rather than a ready-made map. He has been a bit reckless, a bit casual, and has often swerved off on tangents.
The mood is warm, humble and quietly humorous. There's gratitude at the end: after potholes, mud, rain and sleet, he hasn't hit a dead end yet. The tone is hopeful, not boastful.