Oh, weep for Adonais! The quick Dreams, The passion-winged Ministers of thought, Who were his flocks, whom near the living streams Of his young spirit he fed, and whom he taught The love which was its music, wander not— Wander no more, from kindling brain to brain, But droop there, whence they sprung; and mourn their lot Round the cold heart, where, after their sweet pain, They ne'er will gather strength, or find a home again.
The speaker uses a metaphor here, comparing Adonais-Keats to a shepherd tending his flock of sheep. The flock, in this case, is made up of dreams and thoughts.
Keats, as a poet, would have spent much time sorting through his thoughts and dreams before writing them. In the same way, shepherds have to keep their sheep in order, too.
The speaker calls these dreams "Ministers of thought," giving them a place of high honor in the mind.
Adonais-Keats taught them about love, too. That's definitely something poets know a thing or two about, right?
He's dead, though, so the dreams can no longer set others' minds on fire ("kindling brain to brain"). Kindling is used to start fires; the speaker mourns that Keats will no longer be able to spread his dreams to others through his poetry.
In this sudden switch from metaphorical to figurative language, the speaker imagines the dreams being stuck inside the youth's brain, from "whence they sprung" or, where they began. They, too, mourn their fate never to be shared with anyone.
The speaker also imagines the dreams trying to find some life in Keats' heart, but they are disappointed. They will never find a home or any life in Keats again. He's gone (sniff).