Or go to Rome, which is the sepulchre, Oh, not of him, but of our joy: 'tis nought That ages, empires and religions there Lie buried in the ravage they have wrought; For such as he can lend—they borrow not Glory from those who made the world their prey; And he is gather'd to the kings of thought Who wag'd contention with their time's decay, And of the past are all that cannot pass away.
Or, the speaker says, the mourners could just go to Rome. That's where Keats is buried, and there's a "sepulchre" (stone monument) there.
It's not just a monument of Keats, though, says the speaker; it's a monument of "our joy." He goes on to explain:
There are a lot of historical ruins in Rome. It reminds him of wars, empires, and religions, which reminds him of the "ravage" (destruction) mankind "wrought" (caused). But, he says, that's not the reason the mourners should go to Rome. There's no glory in treating the world like prey, the way those empires did, he says.
Oh no. The real glory is present with people like Keats, one of the "kings of thought" fighting against the decay of time ("wag'd contention") with his mind.
Because of these "kings," some things outlive mankind's struggles ("are all that cannot pass away").
Keats is buried among many people like this, and that's the reason that the speaker tells mourners to visit Rome. He thinks, if they see these monuments, mourners will be reminded that death isn't necessarily the end, because great ideas live forever. He thinks that will comfort them. So… get your passports ready, gang.