Amidst the shirred remains of the harvested field, the speaker observes signs of the last remaining days of Autumn that part with the melancholy of the "sober birds" and the leaf that "Comes softly rattling down." He ends his "going forth" by stooping to pick the last aster flower that remains. Albeit faded, the speaker offers it as a symbol of renewal as he carries it "again to you."
Thus, in Frost's poem about the somber approach of winter and the death of the beauty of nature, there emerges the symbol of love in the faded blue of the aster, a flower that will signify life and renewal as the speaker returns to his love. In the midst of death, there is yet life. And, in human relationships, there is a like theme of renewal of love from the solitary, faded aster, the unconquerable spirit.
Another interpretation finds the speaker at the end of his life, which he perceives as completed like the harvest. As he enters the garden, it is a somber moment as he witnesses his life fade. Picking the aster reminds the speaker of his love whom he will soon meet; he is coming "again to you."