The poem begins with a first-person speaker feeling "oppressed," or weighed down, by a "whirl of thought." While the poem doesn't specify what this "thought" relates to, it's tempting to envision the speaker lamenting the sorry state of the world.
The speaker "sink[s] from reverie to rest," slipping from daydreaming to sleep. The alliteration and assonance of "reverie" and "rest" smoothly link these words, evoking just how easy it is to doze off. Yet a "horrid vision" then "seize[s]" the speaker, interrupting that rest. "Seized" is a particularly active verb, signaling aggression and discomfort; the speaker seems to be overtaken by this vision against their will.
The speaker sees the dead spring up from the ground. Specifically, the speaker says that "graves give up their dead." This personification suggests that, upon recognizing the arrival of a greater power (God), the earth itself is relinquishing its control of the human beings buried within it. The lively /g/ alliteration here ("graves give") also makes this apocalyptic image all the more gruesome and vivid.
These four lines establish the poem's meter and rhyme scheme, both of which are typical of Swift and of the Augustan era during which he wrote. The poem uses iambic tetrameter, meaning each line has four iambs (poetic feet that follow an unstressed-stressed syllable pattern, da-DUM).
The first line actually begins with an anapaest ("With a whirl"), however, a subtle variation that immediately conveys disruption and unrest:
With a whirl | of thought | oppressed,
I sink | from re- | verie | to rest.
An hor- | rid vi- | sion seized | my head,
I saw | the graves | give up | their dead.
The speaker also uses rhyming couplets (oppressed/rest, head/dead), which quicken the poem and make its wit seem all the more biting and comic.