Seven short lines, and the mood flips on the sixth. Shelley spends the first five building the most reassuring morning imaginable, then turns on it with a single word: "Laugh." The scene is one of total harmony. Soft winds, sunny skies and a green earth all "harmonize", and the dawn climbs "up the windless heaven" without a care. Then the reason for the laughter arrives, and it is not comforting. Hidden "in the day" are "clouds and whirlwinds" that "watch their prey." The central idea is that beauty and threat are not separate things. The danger is not waiting somewhere beyond the lovely day. It is inside it, ambushed.
The construction makes this turn feel sudden. The poem is one long sentence. The opening "When" clause stretches across five lines before the main verb finally lands, so the reader is held in the calm without noticing how long it lasts. The lines are short, about seven syllables each, with a light, tripping beat that falls heavily on words like "soft", "sunny" and "young". The rhyme is tidy, with a couplet (skies/harmonize), then a run of three (dawn/fawn/gone), then a closing couplet (day/prey). "Gone" sits close to "dawn" and "fawn" rather than matching them exactly, which gives the middle of the poem a slight looseness, as if the morning is drifting upward out of reach. The final couplet snaps shut with a hard, monosyllabic rhyme. "Prey" is a blunt word to end on after so much softness.
The imagery does the quiet work of preparing the trap. Look at the simile for dawn: "Bold as an unhunted fawn." On a first reading it is charming, a young animal at ease in the open. But the word "unhunted" is doing something sly. The fawn is bold only because it has never met a hunter, so its confidence is really ignorance. Shelley plants the vocabulary of hunting before the poem admits it is about hunting. When "ambushed" and "prey" arrive two lines later, we realise the fawn was a rehearsal. The dawn is the unhunted creature, and so, by implication, is anyone enjoying it.
The weather words are also carefully arranged. The poem opens with "soft winds", moves to a "windless" heaven, and ends with "whirlwinds". The wind is gentle, then absent, then violent, and the middle stage is the unsettling one. A windless sky is the stillness before something happens, though the poem only lets us see that in hindsight. The "clouds" are another small shock, because in a sky that is "sunny" and clear, clouds are exactly what we cannot see. That is the point of an ambush.
Then there is the command itself. "Laugh" is hard to place in tone. It could be a bitter warning that the cheerful are fools. It could also be a kind of defiant advice: since catastrophe is hidden in every fine day, enjoy the day anyway. The dashes around "for ambushed in the day" cut into the sentence like a whispered aside, so the speaker seems to lean in and lower his voice before delivering the bad news. The poem does not choose between the readings, and the ambiguity is much of its force. A flat warning would be easy to dismiss, whereas a command to laugh, with menace attached, lingers.
The title calls the poem a fragment, and it does read like a flash of insight rather than a worked-out argument, though the text itself gives no reason to think anything is missing. Its significance lies in how much it packs into so little space. In seven lines it takes a conventional, pretty nature scene and shows how fragile its calm is, using only the poem's own language of hunting and weather. Its pleasant surface turns out to be the cover for the trap, and a reader who relaxes into the first five lines has been caught in the same ambush the poem describes.