Lines 1 and 2 present the main character and topic of the poem: the season of autumn. These personify autumn and begin to characterize its contradictory nature, hinting at one of the poem's main themes: the simultaneous existence of beauty, life, and death. These lines also preview how the rest of the poem will deal with meter and punctuation.
The whole poem is basically an apostrophe to autumn—the title is "To Autumn," and autumn is explicitly addressed in each stanza. In stanza 1, the address identifies autumn as the "Season of mists and mellow fruitfulness." This word choice efficiently characterizes autumn: it is a season whose distinguishing characteristics include its mistiness and "mellow fruitfulness."
These two features work both with and against each other. In one sense, the mist is part of what makes autumn so mellow, hanging over the fields in soft, peaceful silence. On the other hand, the mist is something that conceals. Lurking behind it—and behind autumn's mellow fruitfulness—is another autumnal feature: death. The season's abundant fruitfulness means it has reached its peak. In the rest of the stanza, all the overripe imagery will emphasize this point; the next, unspoken step is decay.
To a present-day reader, this misty, fruitful combination might seem rather clichéd. It's exactly what one might expect to find in pumpkin patch and haunted house imagery for children in October. Instead of perpetuating the cliché, however, the poem uses it to hint at autumn's complexity, something it will be the speaker's job to explore.
Line 2, by adding "the maturing sun" to the list of characters, also gives the first signal that humans will be absent from the poem. Yes, later on the speaker will personify autumn to such an extent that it seems like a real human, but not once will an actual human character enter the scene. The poem will have more to do with natural processes and beings—for example, the friendship between autumn and the sun.
The two are "Close bosom-friend[s]." The words "Close" and "bosom" lend the setting comfort and warmth, and the hyphen linking "bosom" and "friend" represents the inseparable nature of this friendship. Here again, there's a sort of contradiction: the warmth of this friendship is lovely, but the impossibility of altering its outcome—which is decay—is dreadful. The word "maturing" also emphasizes autumn's complexity. By maturing the fruits, the sun brings life, but it also brings them closer to death. The sun itself is also maturing. Its light thins out as earth spins into the low-angled sunlight of winter.
This theme of constant change is also reflected in the poem's meter, which often includes variations. Lines 1 and 2 both follow iambic pentameter, but with irregularities worth looking at:
Season | of mists | and mel- | low fruit- | fulness
Rather than start with an iamb, the line starts with its opposite: a trochee, which is a foot consisting of a stressed-unstressed syllable pair. This trochee gives autumn, the "Season," extra attention right off the bat. It also makes the reader hear more clearly the first syllable "Sea-," which sounds like "see." "To Autumn" is a highly visual poem, loaded with dense imagery. It's about seeing. After this emphasis, the line falls back into its mellow iambic meter.
Line 2 follows a weaker iambic pentameter:
Close bos- | om-friend | of the | matu- |ring sun
All the unstressed-stressed feet are where they need to be, though these stresses aren't as strong as they are in other lines of the poem (see line 4 for a good example). The hyphenated "bosom-friend" contributes to the meter's weakness because it encourages a faster reading that deemphasizes the stress in syllables like "bos-" and "friend." This deemphasis does contribute, however, to a meditative dreaminess that will appear in much of the poem, especially stanza 2. Rather than speaking directly at autumn with the clear meter of someone making an order, the speaker taps into an atmospheric reverie.
This dreaminess, however, will soon narrow into a concrete series of observations in stanza 1. The semicolon that end-stops line 2 marks the transition into a new thought and gets the reader used to the presence of such punctuation. Semicolons, as well as commas, end-stop many lines in the poem, imposing a little bit of order on the poem's overwhelming imagery.