The poem begins abruptly, launching without preface into a list of England's problems. The list is so long and breathless with outrage that the sentence's main verb ("Are") doesn't arrive until line 13! Shelley didn't even title the poem; it acquired the title "England in 1819" when it was posthumously published in 1839 (in a volume edited by the poet's wife, Mary Shelley).
The list starts with an attack on King George III, sometimes remembered as "Mad King George." At the time, George was 81 years old, clinically insane after years of deteriorating mental health, completely blind after years of deteriorating eyesight, and on the verge of death. (Shelley wrote the poem in December 1819; the King died the following month.) Due to his worsening mental illness, the Prince of Wales (the future George IV) had effectively replaced him as England's leader in 1811; George III retained his power in title only.
These conditions explain Shelley's description of the king as "old," "mad," "blind," and "dying." As for "despised," that's Shelley's opinion, but while George's popularity had fluctuated over the course of his lifetime, there's no doubt that his reign had been controversial. For example, he had presided over the loss of the American colonies during the American Revolution decades earlier. More immediately, the political conflicts of 1819, especially the Peterloo Massacre referenced in lines 7-9, had left much of England outraged at the monarchy and aristocracy (more on that massacre later on in this guide).
Also note that the word "blind" here has a metaphorical as well as literal meaning. Shelley suggests that the King (as one of the "Rulers" in line 4 who don't "see," "feel," or "know") can't perceive or understand his country well enough to lead it properly.
The alliteration and consonance of heavy /d/ sounds, plus the appearance of hard /b/, /p/, and/k/ sounds, underscore the poet's anger:
An old, mad, blind, despised, and dying king,—
The assonance of long /i/ sounds in "blind," "despised," and "dying" adds to the line's rhythmic intensity.
Finally, this poem is a sonnet, and follows the standard sonnet meter of iambic pentameter (meaning there are five iambs, poetic feet with a da-DUM rhythm) per line. But from the start, the speaker plays with that expected meter, slotting a spondee (DUM-DUM) into the second foot:
An old, | mad, blind, | despised, | and dy- | ing king,—
These extra stressed beats and sonic devices all combine to create an explosive start to a politically explosive poem.