The inheritors of unfulfill'd renown Rose from their thrones, built beyond mortal thought, Far in the Unapparent. Chatterton Rose pale, his solemn agony had not Yet faded from him; Sidney, as he fought And as he fell and as he liv'd and lov'd Sublimely mild, a Spirit without spot, Arose; and Lucan, by his death approv'd: Oblivion as they rose shrank like a thing reprov'd.
Other poets, "inheritors of unfulfilled renown" (inheritors that didn't get a chance to reach their peak) show up.
They all died young, too, and now sit on "thrones" in the afterlife.
The first is Thomas Chatterton, an English poet that died at 22. He died in 1770 and his agony hasn't "faded from him" yet. In other words, his death is still pretty fresh.
Next up is Sir Phillip Sidney, who died of dysentery in 1586 at age 32 (not a good way to go). He is described as "mild" which means he wasn't very extreme or easily upset. He's also a "Spirit without spot" (without fault).
The final visitor is Lucan, the ancient Roman poet who died at 24. Dude's been dead a long time.
As these poets "rise" they make "Oblivion" (a.k.a. death) shrink and hide as if it has been chastised.
The speaker is saying that these three poets are a good example of how death doesn't mean the end of someone; their work is good enough to live on. In that sense, they cheat death and oblivion—just like Keats.