In the first stanza, the speaker describes the autumn as a harvest time. Everything is bursting with "fruitfulness." Things are so full and ripe that, following the summer, the bees "think warm days will never cease."
In the second stanza, the speaker addresses the personified autumn, noting the moments one sees autumn in the midst of harvest. The tone shifts from the celebratory first stanza to one indicating a sense of rest. As the fruits are harvested, the plants and fields are more desolate. The "half-reap'd furrow" is "sound asleep." Autumn's "hook" (scythe) is the instrument of harvesting the fruits. This hook/scythe is associated with death. The autumn season signifies the end of vibrant life in nature and the approaching season of winter. The last image is of the personified autumn watching the harvested apples processed in the "cyder-press." Autumn is "patient" as if trying to forestall the coming winter.
In the last stanza, the speaker describes the song of autumn. The sounds of the swallows, crickets, and lambs are a "choir" singing the approach of the shortened days and the colder weather of late autumn and winter. The clouds "bar" the sun; the increasingly overcast skies of autumn signify shorter days and less light in general. The decrease in sun light, the "soft-dying" day, is associated with darkness, sleep, and death; all natural progressions.
While barred clouds bloom the soft-dying day,
And touch the stubble-plains with rosy hue;
Then ina wailful choir the small gnats mourn
Thus begins the sounds that signify the end of summer and the oncoming winter. There is an inherent analogy between the cycles of the seasons and the cycle of life with autumn signifying an end or a rest.