Throughout the poem, the speaker addresses autumn as if it were a person. In the first stanza, he notes that autumn and the sun are like best friends plotting how to make fruit grow and how to ripen crops before the harvest. The ripening will lead to the dropping of seeds, which sets the stage for spring flowers and the whole process starting over again. He tells us about the bees that think summer can last forever as they buzz around the flowers. But the speaker knows better.
The second stanza describes the period after the harvest, when autumn just hangs out around the granary where harvested grains are kept. Most of the hard work has already been done, and autumn can just take a nap in the fields, walk across brooks, or watch the making of cider.
In the third stanza, the speaker notes that the music of spring is a distant memory, but that autumn's music is pretty cool, too. This music includes images of clouds and harvested fields at sunset, gnats flying around a river, lambs bleating, crickets singing, and birds whistling and twittering. All of the sights and sounds produce a veritable symphony of beauty.