Here pause: these graves are all too young as yet To have outgrown the sorrow which consign'd Its charge to each; and if the seal is set, Here, on one fountain of a mourning mind, Break it not thou! too surely shalt thou find Thine own well full, if thou returnest home, Of tears and gall. From the world's bitter wind Seek shelter in the shadow of the tomb. What Adonais is, why fear we to become?
The speaker tells us all to pause here, at the graves. (He's basically acting like our own personal tour guide of Rome now.)
These graves are too "young" or new to have "outgrown the sorrow." The people buried there are too recently dead for people not to mourn them yet. That's why the people who visit there, including those that mourn Keats, are still full of "tears and gall" (gall means irritation).
The speaker tells them to set a "seal" on the fountain of their grief, like how someone would stop a fountain from flowing.
Instead, he says, take shelter under Adonais-Keats' monument, which reminds the speaker of eternal life.
There's no reason, he says, to fear being dead. Like he's described in earlier stanzas, death just means becoming part of something lasting.