Keats's "To Autumn" is a poem of description and rich imagery; much of the imagery looks to work against the stereotype of autumn as a time of withering and waning (moving toward winter). As you have already figured out, the first stanza is a straightforward description of nature in autumn--full, lush, and beautiful.
The second stanza of the poem personifies autumn, meaning the speaker describes autumn as if he (or she) were a person. In this stanza, autumn is portrayed as sleeping, either on a granary floor (a storage unit for storing grain), on the rows he is harvesting, or by a cider-press. The notion that autumn is sleeping continues the motif of heavy fullness evident in the first stanza. In addition, the places where autumn sleeps are places usually associated with autumn, such as places of harvest and apple-picking. Significantly, autumn is also portrayed as a "gleaner," which is a worker who harvests crops, again emphasizing the fullness and lushness of autumn.
The third stanza begins by comparing autumn to spring; the speaker seems to suggest that many would prefer spring to autumn, but that they are mistaken, as autumn has it's own unique beauty ("thou hast thy music too"). While the other stanzas used visual imagery to portray autumn, this uses auditory imagery. In listing the "songs" of autumn, Keats is describing all the sounds that characterize the season, such as the bleating of sheep and the singing swallows.
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This poem is an ode, a tribute, like a toast to this season of harvest--"To Autumn." We expect to hear positive things after hearing that title, and we do.
The imagery in stanza one is of ripeness, of a world ready for harvest: "fruitfulness," "maturing," "load and bless," "swell," "plump," and "o'er-brimm'd." Hard to miss this picture of creation as a ripened field ready to harvest.
Stanza two imagery is full of harvest language: "store," "a granary floor," "winnowing," "a half-reap'd furrow," "hook," "swath," "gleaner," "cyder-press," and "oozings." Clearly this picture is one of a harvest either in progress or completed. These images are still full of life, rather than depicting death or emptiness.
Finally, the third stanza asks us not to think of spring (a time of newness and rebirth) as being better than autumn--traditionally a time before winter and the death/hibernation of all creation, including man.
"Where are the songs of spring? Ay, where are they?
Think not of them, thou hast thy music too."
This is also a beautiful time. Note the stubble in the harvested field reflecting the glow of a setting sun; the river swallows who perform as a "wailful choir" as they dip and swoop with the breeze; and the lambs and crickets and robins making their familiar sounds.
This poem is a tribute to autumn, rarely seen--in poetry, anyway--as a time of beauty. While it is a time of reaping what has been sown, metaphorically fall is a precursor to impending death. The narrator asks us to examine the beauty of this season without any looking ahead to the winter which will inevitably come.