As a poet, Keats is probably most famous for his odes. An ode is a poem that addresses a person or object that can't talk back. The form originated in Ancient Greece, where poets like Pindar and Horace sang them at public events, often accompanied by music. Here Keats addresses a season of the year. To use a technical term, it's like one big long case of apostrophe. (Apostrophe is when an idea, person, object, or absent being is addressed as if it or they were present, alive, and kicking.) But the ode is more than just a device; it's a highly structured form with built-in divisions and transitions. Compared to Keats's other odes like, "Ode on a Grecian Urn" or "Ode to a Nightingale," "To Autumn" is shorter at three stanzas of eleven lines each (the other two are five and eight stanzas apiece).
The rhyme scheme of each stanza is ABAB CDEDCCE. You'll notice that this scheme divides the stanzas into a section of four lines and a section of seven lines. The first four lines set up the specific topic of each stanza – ripeness, harvesting, and song, respectively – and the last seven lines elaborate.
The meter of the poem is generally iambic pentameter, in which each line has five ("penta") iambs consisting of an unaccented syllable followed by an accented one:
A-mong | the riv|-er sal|-lows bourne | a-loft.
But the iambic pentameter is more like the default setting than a strict requirement in "To Autumn." Maybe critics have marveled at this poem's complicated use of many different kinds of rhythm. For example, the beginning each stanza departs from the iambic meter in the same way: with an accented syllable right off the bat: "Sea-son," "Who," and "Where." You could write an entire paper on the meter alone, never mind the meaning.