Scorn and despair, — these are mine empire: —
More glorious far than that which thou surveyest
From thine unenvied throne, O Mighty God!

[…]

No change, no pause, no hope! Yet I endure.
I ask the Earth, have not the mountains felt?
I ask yon Heaven, the all-beholding Sun,
Has it not seen? The Sea, in storm or calm,
Heaven's ever-changing Shadow, spread below,
Have its deaf waves not heard my agony?
Ah me! alas, pain, pain ever, for ever!

Poor Prometheus is tied to a rock by Jupiter, the god he's defied to bring fire to mankind—he's being tortured by Jupiter for his defiance. Here, poor Prometheus is reflecting on how much pain he's in.

Prometheus—as imagined by Shelley—is one of the great Romantic heroes. The guy not only defied the gods (Jupiter specifically) to bring fire to mankind, he also ended up being chained to a rock and tortured as punishment for his defiance.

And yet Prometheus endures his pain and torture very heroically. "Pain and scorn" may be his empire, but they are better than the empire that the tyrant Jupiter oversees. So Prometheus is heroic not only because of his act of bringing fire to mankind; he's heroic because he endures the (very, very terrible) consequences. He's courageous, and rebellious, and stoic all at once.

We get a good sense of the pain that Prometheus is enduring here through his rhetorical questions: "I ask the Earth, have not the mountains felt?" This is huge pain that poor Prometheus endures, and we can see it not only in the repeated questions, but in the repetition of the last line: "Ah me! Alas, pain, pain ever, for ever!"

All of this repetition is emphasizing the extent of the torture that Prometheus has to put up with. And it gives us a sense of what a hero he is, dealing with all this pain.