Season of mists and mellow fruitfulness, Close bosom-friend of the maturing sun; (lines 1-2)
The first line distinguishes autumn from the other seasons, while the second line sets up the personification of autumn throughout the poem. Why would the sun be friendlier with autumn than with any of the other seasons? Oh, spring is going to be so mad when she hears about this.
Conspiring with him how to load and bless With fruit the vines that round the thatch-eves run; (lines 3-4)
The first stanza gives an optimistic view of nature plotting to help people. Moreover, the imagery integrates plants and human dwellings.
Sometimes whoever seeks abroad may find Thee sitting careless on a granary floor, (lines 13-14)
Tracking down autumn is like trying to track down a leprechaun. Autumn has a few choice hideouts (sadly, not at the end of rainbows), but she's always on the move. She has no cares once the work of harvesting is finished.
Where are the songs of Spring? Ay, where are they? (line 23)
Keats makes a reference to the pastoral tradition that started in Ancient Greece. Within this tradition, shepherds piped simple songs in honor of the seasons. He may even be referring back to his own famous "Ode on a Grecian Urn," in which a young Greek man pipes a tune beneath a tree.