Four lines, one sentence, and a title that gives the game away. Shelley's "Fragment: A Tale Untold" is about a story that never gets told, and it is itself a story that stops almost before it begins. The poem is so small that every word has to carry weight, and its two images carry a surprising amount of disappointment.
The first line is grammatically slippery. "One sung of thee who left the tale untold" can be read two ways. Either the person addressed as "thee" left the tale untold, or the singer did. The poem never decides, and it never says who "thee" is, so any specific story behind the line would be guesswork. What the line does establish is simple: a song was begun, a tale was promised, and it was not finished. The rest of the poem tries to say what that feels like.
It does so through two similes, introduced by the repeated "Like." The first is "the false dawns which perish in the bursting." A false dawn is a glimmer of light that looks like the start of day and then fades. The verb "bursting" suggests a dawn breaking triumphantly open, but it also suggests a bubble, and what bursts, dies. So the beginning and the end happen in the same instant. The light does not fade slowly. It perishes in the very act of arriving, which is a neat description of a tale that stops as it starts.
The second simile moves from the eyes to the mouth. "Empty cups of wrought and daedal gold" is a richly built phrase. "Wrought" means worked by hand, and "daedal" comes from Daedalus, the legendary craftsman, so these cups are intricate and beautiful. The sounds pile up, and then the adjective "empty" sits in front of all that gold and undercuts it. The cups do worse than fail to satisfy. They "mock the lips with air." Mockery is active, almost cruel, as if the beautiful object were teasing the person who reaches for it. The disappointment is also more physical than in the first image. Watching a dawn fade is sad, but lifting a cup to your lips and tasting nothing is a small bodily shock. "Lips" may also quietly echo the singer of line one, since lips are what sing and what drink, and both are left wanting.
The sound of the poem supports this. The rhyme scheme is ABAB, pairing "untold" with "gold" and "bursting" with "thirsting." The lines are roughly iambic pentameter, but the rhymes behave differently. "Untold" and "gold" are single stressed syllables ending in a long, hollow *o*, which has a closed, metallic ring, like tapping a cup. "Bursting" and "thirsting" have an extra unstressed syllable on the end, a so-called feminine ending, so those lines trail off instead of landing. Both words also share a harsh *-urst-* sound, which makes the poem's roughest consonants fall exactly where the images fail. The final word, "thirsting," is the last thing we hear. The poem ends on desire with nothing to satisfy it.
That is where its significance lies. The title calls the poem a fragment, and the poem is exactly what it describes. A reader who finishes it wanting more, wanting to know who "thee" was and what the tale contained, is the thirsty one, and the poem is the golden cup that mocks with air. We cannot say from the text whether Shelley meant it as a finished miniature or as a piece left incomplete, but the effect is the same either way. Form and meaning have become one thing.
The poem also shows how much can be packed into very little space. It has no narrative, no named character, and no explanation, only two pictures: a light that dies as it rises and a gorgeous vessel with nothing inside. Together they make absence feel almost like a presence. Shelley is one of the major English Romantic poets, but a reader needs no background to feel the poem's point. Most people know what it is to be promised something and left holding the empty cup.