To that high Capital, where kingly Death Keeps his pale court in beauty and decay, He came; and bought, with price of purest breath, A grave among the eternal.—Come away! Haste, while the vault of blue Italian day Is yet his fitting charnel-roof! while still He lies, as if in dewy sleep he lay; Awake him not! surely he takes his fill Of deep and liquid rest, forgetful of all ill.
Death is imagined as a king, ruling over things both beautiful and decaying, like Adonais-Keats, who now has a special place in the kingdom of the dead.
He says that Keats "bought" his grave. What did he use to buy it? His "purest breath." This description is another way Shelley characterizes the youth as young and untarnished.
He's also saying that it was this purity that killed him. In the case of Adonais, Artemis' jealousy caused the youth's death, much like (in Shelley's opinion) jealousy over Keats' poetry led critics to write vicious reviews.
Shelley uses some vivid imagery with "blue Italian day." He wants us to picture clear, bright skies above Keats' grave.
The speaker is emphasizing that he's not yet buried deep underground; his death is still fresh. In fact, he looks like he's in "dewy sleep" instead of dead.
So, why emphasize that the youth is newly dead? The speaker does this to get us to take some type of action.
He tells us leave the dead youth alone ("Come away!"), to make "haste" and not disturb him, lying peacefully and having forgotten "all ill," or all the bad things that were done to him.