Shelley's speaker is desperate, and the first word, "pant," makes that physical at once. The poem never describes a melody or says what instrument is playing. It describes what music does to a person who has gone without it: it works like water on someone dying of thirst. Its central idea is that music is a necessity, restoring the heart and releasing it from "care", and it can be as intoxicating as wine or desire.
Nearly every image comes from the same family of thirst and drink. The heart is "a dying flower," the speaker a "herbless plain" waiting for "gentle rain," and the music is "enchanted wine" poured out. The sound is turned into liquid, then turned back into weather, as notes fall in "a silver shower." This blending of hearing, taste and touch is why the poem feels so sensuous. When the speaker begs, "More, oh more,--I am thirsting yet," the cry breaks the metre's politeness, and the reader hears an addict's craving as much as an art lover's.
The second stanza darkens the picture with a different kind of image. Care has bound a "serpent" upon the heart "to stifle it," and the music "loosens" it. The verb quietly echoes the first stanza, where the singer is asked to "loosen the notes." The same release that frees the sound frees the listener. The stanza ends by tracing the music's path inside the body: the "dissolving strain, through every vein, / Passes into my heart and brain." The internal rhyme of strain, vein and brain makes the sentence flow smoothly, so the sound seems to pass through the body as the lines are read.
Structurally the poem is steady and song-like. Each of the first three stanzas has six lines rhyming ababcc, with the final couplet shortened and tightened, as if the speaker is running out of breath. The rhythm is a quick, lilting triple beat, closer to a dance than a lament, which suits a poem about being carried along. In the first stanza the couplet "Like a herbless plain, for the gentle rain, / I gasp, I faint, till they wake again" has its own internal rhyme (plain, rain), and the short phrases "I gasp, I faint" almost enact the fainting they describe.
The third stanza turns to a longer image, a violet withering beside a lake after "the hot noon has drained its dewy cup." The irony is that the flower dies beside water and still goes thirsty. Its scent escapes anyway, "on the wings of the wind o'er the waters blue." Judging by the imagery, the violet may stand for the speaker's heart, where the body fails but something fragrant and musical lives on. The stanza also sets the speaker's longing against a cruel, drained landscape, and the "cup" it empties prepares for the next image.
The final stanza changes the mood. A "charmed cup" of "foaming, and sparkling, and murmuring wine" is filled by "a mighty Enchantress" who "invites to love with her kiss divine." Music, first a rescuing rain, becomes a seductive woman, and relief becomes temptation. The word "divine" also returns from the poem's first line, where music itself was divine. Pleasure, beauty and something close to the sacred are folded into one thing. As given here, the text breaks off on an ellipsis, and the similes in the last two stanzas never reach a main clause. The poem seems to end in the middle of its own thought, still reaching for the comparison that will explain the music. That unfinished quality suits a poem about thirst, because it ends without being satisfied.
The poem matters because it shows what makes Shelley, one of the major English Romantic poets, so distinctive. He does not argue that music is powerful. He makes the reader feel it by turning an invisible sound into rain, wine, scent and a kiss. In a few short stanzas it also shows how closely the Romantics linked art with emotional survival, the sense that without beauty the heart simply withers.