Season of mists and mellow fruitfulness, (line 1)
Autumn is the most subdued and least flashy of the seasons. That's why they call her mellow yellow... er... "mellow fruitfulness." Though we do tend to associate autumn with the color yellow.
And fill all fruit with ripeness to the core; To swell the gourd, and plump the hazel shells (line 6-7)
The language dives in to the very center of the fruit, only to push outward again like a balloon. Autumn is the secret ripeness expanding from the center of things.
Thee sitting careless on a granary floor, Thy hair soft-lifted by the winnowing wind; (lines 14-15)
A granary is where grains like wheat are stored. This is the only image in the poem that seems to eroticize autumn, as her hair is blown to and fro by the wind.