First, let's review what "personification" means. Personification is when an author gives an inanimate object or abstract idea human qualities. In John Keats' poem "To Autumn" the poet begins to personify the season of autumn at the beginning of the poem. In the line "Season of mists and mellow fruitfulness, / Close bosom-friend of the maturing sun;" Keats personifies autumn ("season of mists") by calling it a "bosom-friend," a term usually reserved for humans. A typical way to personify an object/idea is by applying human verbs to it (for example, "the howling wind" or "the raging storm" use verbs to animate elements of the natural world). Throughout the poem, Keats continues to personify autumn by applying human verbs to autumn. For example, the season conspires with the sun (line 3) and later in the poem, sits (line 14) and drowses (line 17).
In the second stanza, the animation of autumn becomes more explicit: Autumn is characterized as if it were a person with human-like adjectives and nouns:
"Sometimes whoever seeks abroad may find
Thee sitting careless on a granary floor,
Thy hair soft-lifted by the winnowing wind;
Or on a half-reap’d furrow sound asleep,
Drowsed with the fume of poppies, while thy hook
Spares the next swath and all its twined flowers:"
Autumn is no longer an abstract season: she is a person asleep on the floor with her hair lifted by the wind. This is a literal example of personification: Autumn has a head, hair, and body, like a person. The last stanza of the poem zooms out to the cycle of the seasons, and focuses less on Autumn's personification. The key things to notice are this: the use of active, human verbs like "conspire", "bless", "sit" "sleep" and "watch" as well as the use of concrete words that show autumn as a person: her head, hair. It's a beautiful poem: enjoy!