A figure of speech occurs when a word or groups of words have a resonance beyond their literal meaning. Keats, for example, uses the ubi sunt or "where are they" figure of speech when he asks, in the third stanza, "Where are the songs of spring? Ay, Where are they?" This is not question meant to be answered in a literal way, but a form of lament for days gone by in which the poet ponders death and the passage of time.
The poem abounds in images. Autumn is likened in the first stanza to something pregnant giving birth to abundance: gourds "swell" and the hazel-nuts "plump" or grow full. The cottage trees are so full they "bend with ripeness" and all the fruits experience "ripeness to the core." Nature is fecund, maturing, and delivering its bounty.
Autumn's nature is so fecund it is sleepy from its overabundance, as a person might be who has eaten too much. Autumn is personified as "sound asleep" and "drowsed."
Not only autumn, but various creatures and machines in it are personified: the bees "think" as humans might that "warm days will never cease," whereas the "cyder press," like a human, "watchest the last oozings" as the apples are pressed into juice, and the gnats "mourn" in a "wailful choir."
Keats uses words that conjure slowness to evoke the slowing down that autumn represents to him: the "clammy cells" of the bees make us think of thick, slow-moving honey and "oozings hour by hour" conjure the slow pressing of the apples by the cyder press.
Keats also uses alliteration in the pile-up of "s" sounds: "cease," "summer" and "cells," "seen," "sometimes," "seeks," "sitting," "soft," "sound," and "swath," all of which lend a sleepy cadence to the poem.
All in all, the sounds and images in the poem reinforce the idea of autumn as a sleepy time of slowing down as nature's bounty comes to fruition.