Keats's ode "To Autumn" is an apostrophe, which means it directly addresses someone or something absent or merely rhetorical. In this case, as the title of the poem indicates, the speaker addresses the season of autumn:
Season of mists and mellow fruitfulness,
Close bosom-friend of the maturing sun (ll.1-2)
The image describes the close friendship of autumn and the sun that has helped the crops to grow; together, they have "conspired" to produce the abundant harvest detailed in the rest of the first stanza.
In establishing a personal relationship between inanimate entities, Keats introduces the personification of autumn that will run throughout the poem. Indeed, his use of apostrophe already endows the season with a degree of humanity, but it is enhanced in later stanzas as we see him sitting on the floor amidst the grain, napping in a meadow, and engaging in various harvest-time tasks. These actions depict autumn as a productive figure who can nevertheless stop to appreciate the beauty around him in the "winnowing wind" (l.15) or the "fume of poppies" (l.17).
This picture of a vibrant individual seems meant to contradict the hint of anxiety the speaker feels at the waning of the year that autumn brings, and the coming of winter, the metaphorical season of death. This hint is already present in the "maturing" sun of the second line - autumn and his best friend are not exactly young anymore. Yet the speaker urges autumn, "Think not of [the songs of spring], thou hast thy music too" (l.24). Although autumn is not the time of new birth symbolized by spring, the poem insists on the value of its different beauty. This assertion is perhaps especially poignant considering that this was one of the last poems Keats wrote before his early death of tuberculosis. So near the end of his life, the poem articulates the beauty and richness still to be found in maturity and endings.