He will awake no more, oh, never more! Within the twilight chamber spreads apace The shadow of white Death, and at the door Invisible Corruption waits to trace His extreme way to her dim dwelling-place; The eternal Hunger sits, but pity and awe Soothe her pale rage, nor dares she to deface So fair a prey, till darkness and the law Of change shall o'er his sleep the mortal curtain draw.
Death has come and taken him; he won't be waking up ever again. Sad.
Death is described as "white," which is not usually the way we picture the Grim Reaper (usually the dude is shrouded in black). Is Death described as white because Adonais-Keats has been described as "pure" earlier in the poem? Does that mean that he is being taken to a heaven? We'll see, but it's a good thought to keep in mind for now.
Corruption and Hunger (abstract concepts that get personified here) wait outside the door.
These two basic, human things can't seem to reach our youth. They are in awe of him, but they also pity him.
Shelley is saying that Keats was too good to be corrupted.
They also don't want to "deface" or disfigure such a beautiful man, even though the speaker describes him as their "prey." Shelley has given Adonais-Keats such beauty that even other-worldly powers don't want to mess with him—that is, until "darkness and the law of change" (a.k.a. death) comes for his body.
After nature (the "law/ Of change") takes its course, he's no longer anything but a corpse and "the mortal curtain" is drawn, signaling the metaphorical end of his life.