Keats is using personification throughout this poem as he compares the changes in the seasons to the relationship between a woman (autumn) and a man (the sun). In this poem, Keats is looking back at the history behind "her" and recognizing the changes that are heralding the approach of more changes. While not directly addressed, the passage from birth through life and on to death is present throughout the poem.
Autumn is involved with "the maturing sun" in creating all the natural fruits that develop during spring, "conspiring" together to "fill all fruit with ripeness to the core; to swell the gourd, and plump the hazel shells with a sweet kernel."
Throughout the summer, autumn often enjoys the produce of her nature. She sleeps with "the fume of poppies" as her hair is gently blown "by the winnowing wind."
As autumn progresses, however, things change. The third stanza speaks of "barred clouds" and "a wailful choir" of gnats that "mourn." Spring's lambs are now "full-grown" and must now appreciate the song of autumn; the season of spring is past and done. The approach of winter, and its unspoken association with death, is part of the natural cycle portrayed throughout the poem.