The speaker stands in the woods in early autumn and watches the color start to change. Red berries on the dogwoods are only the beginning, because the maples are about to turn too. The speaker pictures God as a painter, dipping a huge hog bristle brush into three shades of fire and sweeping it across the foothills in wide strokes. It's a playful, homespun image, and it makes the season feel like something done with confidence and a bit of swagger.
The poem then looks ahead to rifle deer season, when most trees will be bare and sunlight will finally reach the forest floor. The speaker asks a quiet question: the cooling shade only disappears once we no longer need it. Leaves fall because summer's work is done, and the trees give back to the earth. The closing line, "boy, does it go out in style," sounds like casual talk, and that tone gives the mood its warmth. The feeling is admiration and gentle acceptance, not sadness.