The poem doesn't waste any time getting to its main subject: the beauty and bounty of Autumn. In fact, the speaker begins by addressing the season itself, here personified as a godlike male figure. The first three lines use apostrophe, a direct address: the speaker stops Autumn in his tracks and asks the season to "sit" for a while. It's as though the speaker admires Autumn and wants to be in his company, imploring him to "pass not."
The first few lines characterize Autumn as a time of plenty. It's that point in the year (or at least, it used to be!) when people harvest their crops and celebrate their bounty. Autumn, the speaker says, is "laden with fruit" and "stained / With the blood of the grape." Both images evoke growth, beauty, and richness. But they also suggest a kind of seasonal release: Autumn represents the culmination of a hard few months' work, an ending.
The metaphor of the "blood of the grape" refers to wine, setting an earthy, bodily (and perhaps Eucharistic) tone that will run all through the poem. The idea that this wine will "stain" also suggests spilled cups and drunken revelry. To this speaker, there's something both spiritual and lusty about Autumn.
These first three lines are full of caesurae and sound patterning:
O Autumn, || laden with fruit, || and stained
With the blood of the grape, || pass not, || but sit
Beneath my shady roof [...]
The /d/ and /n/ consonance and long /ay/ assonance here creates a sonic feast: these lines overflow with harmonious sounds. The caesurae contribute to this effect, making the lines feel like they might spill out of the poem's form—its container—and thus evoking both abundance and indulgence.