Who mourns for Adonais? Oh, come forth, Fond wretch! and know thyself and him aright. Clasp with thy panting soul the pendulous Earth; As from a centre, dart thy spirit's light Beyond all worlds, until its spacious might Satiate the void circumference: then shrink Even to a point within our day and night; And keep thy heart light lest it make thee sink When hope has kindled hope, and lur'd thee to the brink
The speaker addresses anyone who is still mourning Adonais-Keats. Come forward, he says, and "know" both yourselves and the youth. Er, say what?
Stay with us, here. Shelley has some pretty weird requests in this stanza. His speaker tells the mourners to "clasp" (touch) the "pendulous" (hanging loosely) earth. Basically, he wants them to touch the ground.
Then their spirit will "dart" away and go somewhere mystical, "beyond all worlds." It'll keep doing that until the "void" (their grief) is "satiated" (satisfied). Then, their souls will shrink down into a tiny point back on earth.
This exercise will keep the mourners' "heart light" and happy, and keep them from losing hope or going "to the brink" (to the edge) of sanity with grief.
So, what was that all about? Basically, he's telling those that still mourn Adonais-Keats to get a hold of themselves and consider how huge the universe it. He thinks it'll help them keep a sense of perspective.