A speaker looks up at an evening star shining through thin clouds and speaks to it as a gentle presence. The night is hushed and the lake is still. The only sound is the murmur of lovers, carried on a soft breeze. The star, the speaker suggests, quiets "the slaves of interest" with its "mild, pitying gaze". Then comes a wish: to look into its beam until "every bond of sense / Became enamoured". The central idea is surrender. Gazing at something beautiful and calm loosens the ordinary hold of the senses and turns it into love.
The syntax carries much of this mood. The first six lines are one drawn-out address to the star, packed with qualifying phrases, so the reader drifts through the description before reaching a main point. "Sweet star" returns at line seven like a refrain, as if the speaker can't stop repeating the name. The grammar also delays the speaker. The first "I" appears only at line thirteen ("Oh, I would look"), so for most of the poem the scene seems to exist without anyone watching it. When the speaker does appear, it is as someone ready to dissolve into the light.
The imagery plays light against dark, with the light always small and the dark always large. The star is a "spanglet of light" on "evening's shadowy veil", a sequin sewn onto cloth, which makes night feel like something woven and worn. The same veil "shrouds the day-beam from the waveless lake". A shroud is a burial cloth, so the day is laid quietly to rest. The star flies "through fleecy clouds", though it is surely the clouds that move. Judging by the imagery, the speaker is already half-dreaming. The evening star is also called "more sweet / Than the expiring morn-star's paly fires". Even the dying morning light, with its pale fire, is a poorer thing than this calm.
The sound of the verse works the same way. It is blank verse, unrhymed lines of roughly ten syllables, with a few stretched to eleven, as in "silvery radiance fliest". Without rhyme to close each line, the sentences spill over the line-ends ("more sweet / Than…", "art thou aught but / Lulling…"), and the effect is of breathing out slowly. Soft, hissing sounds gather where the poem describes quiet: "wearied Nature sinks to sleep", "Sighs in the ear of stillness". The phrase "ear of stillness" is a small paradox, since a sigh can only be heard where silence has made room for it. "Soft Favonius" is the classical name for the west wind, and the poem treats it like a person, whispering to the night.
The poem has a harder edge as well. Among all the "sweet", "balmy" and "soft" words, "slaves of interest" stands out. Slaves is a harsh word, and I read "interest" here as self-interest, the worry over gain that keeps people anxious. The star's "pitying" gaze suggests it sees something sad in them. That makes the closing wish more pointed. The "bonds of sense" suggest ties that fasten us to the practical, calculating world, and if those bonds become "enamoured", restraint turns into tenderness. This is an inference, but it fits the contrast the poem sets up between a world of bargaining and one of love and rest.
Then the poem simply stops. As printed here, it ends mid-sentence on a dash, right where the senses are about to give way. Whether the break is deliberate or not, it suits the subject, because the reader is left in the same suspended state as the speaker.
The poem's significance lies less in any grand statement than in what it shows of its author. Shelley is one of the major English Romantic poets, and the piece is a compact example of the Romantic habit of addressing nature directly and using it to explore feeling. The diction is richly decorated and old-fashioned ("darksome", "paly", "spanglet"), and it can feel overdressed. But beneath the ornament is a real tension between a restless, self-interested world and the wish to be absorbed in something beautiful and still.