Stanza 1 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Season of mists and mellow fruitfulness,
Close bosom-friend of the maturing sun;

From the title it's clear that the speaker is talking about autumn. The speaker briefly describes the season and immediately jumps into personification, suggesting that autumn and the sun are old pals.
"Mists" often accompany chilly weather because the moisture in the air condenses into a vapor when it's cold.
"Mellow fruitfulness" sounds like something people would say at a wine tasting, doesn't it? "Mmm...this season has a mellow fruitfulness, with just a hint of cherry and chocolate." The word "mellow," meaning low-key or subdued, is a good fit for autumn, with its neutral colors and cool, yet not cold, weather. And it's also the season when many fruits and other crops are harvested, making autumn fruit-full.
Autumn is a close friend of the sun, who is "maturing" as the year goes on. "Maturing" could be a polite way of saying "getting old." The sun is no longer in its prime.
A "bosom-friend" is like that friend you told all your secrets to in junior high school.

Conspiring with him how to load and bless
With fruit the vines that round the thatch-eves run;

Ah, so now the sun and autumn are "conspiring," eh? Looks like we might have to separate the two of them. What are they whispering about over there?
OK, so not quite as thrilling as we thought. They are planning how to make fruit grow on the vines that curl around the roofs ("eves") of thatched cottages.
The image highlights the weight of the fruit as it "loads" down the vines.
Thatched cottages suggest a pastoral setting, characterized by shepherds, sheep, maidens, and agriculture. The "pastoral" as a literary genre was thought to originate in Ancient Greece, and the ode is a Greek form, so it is appropriate for this ode to include pastoral themes. Keats's other Great Odes, especially "Ode on a Grecian Urn," include similar imagery.

To bend with apples the moss'd cottage-trees,
And fill all fruit with ripeness to the core;

Keats is going nuts (pun!) with images of weight and ripeness. The richness here is like Willy Wonka's Chocolate Factory set in an orchard.
The apples "bend" down the branches of mossy trees with their weight. The trees belong not to some big farming cooperative, but to the simple cottages of country folk.
The ripeness penetrates deep to the very center of the fruit. They're not like those apples that look delicious until you take a bite and realize that the fruit is hard and sour. No, these babies are ready for chow-time right now.

To swell the gourd, and plump the hazel shells
With a sweet kernel; to set budding more,

In line 6, the ripeness converged on the center of the fruit. Now, the ripeness expands like a balloon to "fill up" nuts and gourds. The opposition of these motions helps us visualize the process.
"Gourds" include things like squash, zucchini, and, especially, pumpkins! What could be more appropriate for autumn than huge pumpkins ripening on the vine?
"Hazel" is a plant that produces the nuts that add delicious flavor to coffee or gelato. The nut is the "sweet kernel" that we eat.
It's almost as if the speaker is coordinating the growth of all these fruits and nuts. He's like, "more! More! MORE!"

And still more, later flowers for the bees,
Until they think warm days will never cease,
For summer has o'er-brimm'd their clammy cells.

The "budding" that the speaker describes is in the future. He has just been describing the "kernels" or seeds that drop to the ground when nuts fall from trees.
These seeds will "later" turn into new plants and flowers when spring comes again.
Autumn isn't just a time of things dying off, turning brown, and falling to the ground. It also sets the stage for the return of growth in the spring. From nature's perspective, fruit is the mechanism for planting new seeds.
The speaker goes on a little imaginative trip into the next spring and summer, where the bees take advantage of the flowers that began as a small seed in autumn. Unlike humans, who can make sense of past, present, and future, the bees only know their task for the present. The bees think the summer will never end, and that the flowers will always be in bloom.
The bees are like monks or prisoners inside of "clammy cells," the cells being the moist insides of the flowers in which they seek nectar.
At this point, even the speaker must admit that all this growth has become too much, and summer is like a sweet liquid that threatens to spill over the brim of a glass. Besides, he is starting to get away from the point. Must be time for a new stanza.