Most musical of mourners, weep again! Lament anew, Urania! He died, Who was the Sire of an immortal strain, Blind, old and lonely, when his country's pride, The priest, the slave and the liberticide, Trampled and mock'd with many a loathed rite Of lust and blood; he went, unterrified, Into the gulf of death; but his clear Sprite Yet reigns o'er earth; the third among the sons of light.
Now he wants Urania to mourn loudly. Sheesh, speaker, make up your mind already.
Since Adonais was a male child ("Sire") of the gods, he was from a "strain" of "immortal" beings. But, apparently Adonais was mortal; after all, he did die.
Keats wasn't descended from immortal gods, obviously, but the speaker elevates him above "normal" mortals—perhaps because he was a poet?
The speaker also calls him "his country's pride," but it doesn't matter. He dies anyway, the same way that priests, slaves, and "the liberticide" (fighters and those who destroy liberty) die: mocked by others.
These "others" are the critics, whom he blames for Keats' death. He calls their criticism a type of bloodlust (mock'd with many a loathed rite / Of lust and blood), the same type of bloodlust that the boar had for Adonais when it killed him.
But, the speaker says, his spirit ("Sprite") lives still. In Keats' case, the spirit is his words. They live on after his death; in fact, the speaker says that they "reign" over the earth, which shows how powerful he thinks Keats' writings were.
In fact, our speaker is such a fan of Keats that he thinks of him right up there with some of the greats. Calling him the "third among the sons of light" refers to Shelley's poet-rating system he explained in "A Defense of Poetry", which listed Homer as the first epic poet and Dante as the second epic poet.