These lines open the section on "Autumn" in Thomson's The Seasons. As the narrator moves from praising summer to praising fall, he pictures autumn as crowned or topped with a sickle, a classic representation of harvesting fall's bounty, and with the "wheaten sheaf" or ripe stalks of grain now ready to be harvested. He personifies the season as a nodding and "jovial" (happy) person. The speaker then turns to describing readying himself for the task of writing about this season, saying he is glad to tune his "Doric reed" (a Greek flutelike instrument) to make the music of his verse. In referring to his poetry as a Doric reed, he is using a metaphor, comparing his words to the music of the ancient world.
All of this sets the scene for what is to come. Thomson, following Virgil's Georgics, praises the harmony of nature's ways. As the verses to follow will imply, in the eighteenth century, as in the ancient world, a country honed its strength through the hard work of the harvest, while the plentiful harvest itself was a sign of God's favor.