The breath whose might I have invok'd in song Descends on me; my spirit's bark is driven, Far from the shore, far from the trembling throng Whose sails were never to the tempest given; The massy earth and sphered skies are riven! I am borne darkly, fearfully, afar; Whilst, burning through the inmost veil of Heaven, The soul of Adonais, like a star, Beacons from the abode where the Eternal are.
The speaker thinks he "invoked" (summoned) this force with his "song." Since he's been using music as a metaphor for poetry throughout the poem, we can assume he means that this poem has summoned the force.
It "descends" on him and makes his spirit leave the world (a "bark" is a boat), far from the crowd (the "throng" who were never tested by to a "tempest," or storm). His spirit's flight makes the "massy" (big) earth and "sphered" (round) skies "riven" (torn apart).
Basically, he's imagining being dead and having his soul carried up to the heavens.
This isn't the most fun trip, though. He is carried ("borne") fearfully and darkly. But, he can see the soul of Adonais-Keats "burning" like a star through the "inmost veil of Heaven." It's shining like a beacon.
When he arrives, he'll be with Keats and the other "Eternal" souls. The poem ends with the imagery of the speaker being reunited with the one he mourns. Good times?