Most musical of mourners, weep anew! Not all to that bright station dar'd to climb; And happier they their happiness who knew, Whose tapers yet burn through that night of time In which suns perish'd; others more sublime, Struck by the envious wrath of man or god, Have sunk, extinct in their refulgent prime; And some yet live, treading the thorny road, Which leads, through toil and hate, to Fame's serene abode.
Keep on mourning, says the speaker (so… what else is new?).
Not everyone was as brave as the youth, who climbed to a "bright station" in life. That means we should mourn him extra-hard.
Not everyone is brave enough to dedicate their life to poetry ("that bright station"), either, he says.
But those who really "go for it" are the happiest, even though they know they'll die someday. The speaker uses a metaphor to compare their lives to candles ("tapers") that burn throughout time. The candles will burn out someday, just like our lives will end someday.
It's also brave, he says, to continue to live this way despite the envy of others, who will try to take them down in their "refulgent" (brightest) "prime."
The haters don't stop everyone, though. Some of these brave folks make it past the envy of others, past the "toil" (hard work) and "hate" of the world, to achieve fame. Hey, this sounds like Keats, who sadly isn't around to enjoy it.