Six lines are all there is, and in them Shelley does one thing: he turns a voice into a river and himself into a boat. The speaker is listening to someone sing, and the effect is more than pleasant. It transports him, and his "spirit" is carried "far far away into the regions dim / Of rapture." The central idea is surrender. Faced with beautiful music, the speaker doesn't steer, analyse or even sit still. He is swept off, a passenger in his own experience.
The vocabulary of the first line sets this up. A "bark" is an old word for a small ship, and "charmed" works twice. It means enchanted, and it also carries the sense of being sung to, since the word "charm" comes from the Latin for song. The boat is under a spell and has no one at the helm. Then there is the verb. The spirit does not sail or row but "swim[s]," as if it were a living creature floating without effort. The singing, meanwhile, is "liquid," a word that describes smooth sound and flowing water at once. Shelley never says the voice is like a river. He lets one adjective blur the two, so by the time the waves arrive, we have already accepted them.
The form does some of the carrying too. The rhyme runs *swim / singing / dim*, then *winging / river / swinging*, which is the pattern of terza rima, the interlocking scheme Dante used and Shelley later used in "Ode to the West Wind." In that pattern the middle rhyme of one three-line unit becomes the outer rhyme of the next. Here the chain is visible in *singing*, *winging* and *swinging*, three present participles that tie the two tercets together and give the sense of motion that never settles. Most of the lines end on an unstressed extra syllable, as in "singing" and "river," so they lap over rather than land with a thud. The metre is a steady iambic pentecameter, but the soft endings keep it from marching. The stanza break works the same way. The sentence slips straight over the gap, "regions dim // Of rapture," so the reader is carried across white space just as the spirit is carried across water.
The second tercet does not bring a new comparison. It gives the boat a landscape. The first image is already a boat on waves, and now we get "a boat, with swift sails winging" its way down a "many-winding river." The picture is repeated and enlarged, which suits a poem about being swept along, since the poem itself seems caught in the current and circles the same image. The sounds help. "Swift sails winging" is thick with s and w sounds, and the verb "winging" slips a bird into the picture, so the boat seems almost to fly. The compound "many-winding" is a long, bending word that makes the line curve as it reads.
It is worth noticing how dark the scenery is. The regions of rapture are "dim," and the river runs "through dark forests." Judging by this imagery, the ecstasy is not bright and clear but shadowy, a state in which the listener loses his bearings. Perhaps that is a faint note of unease under the delight, or perhaps it is simply how Shelley pictures a mind giving itself up to feeling. Either way, rapture here means being lost, and being lost is presented as a joy.
What makes the fragment worth reading is its economy. In six lines it holds a metaphor, an extended simile, a demanding rhyme scheme and a small drama of surrender, all without mentioning who is singing or what the song is. The title marks it as unfinished, so we cannot say where Shelley meant it to go. The closing ellipsis, though, suits the poem's meaning almost too well. The boat is still moving when the words stop, and the reader, like the speaker, is left drifting.