A kiss is supposed to be a gift, and these six lines turn on the discovery that the speaker has kept it for himself. While kissing Helena he feels his soul rise to his lips alongside the kiss. It has come "to follow where the kiss should guide it", which is to say, to cross over to her. Instead he holds it back. The poem ends in self-reproach, not rapture: "Oh, cruel I, to intercept it!" The central idea is a small, neat paradox. Love asks for the whole self, and the speaker blocks that surrender at the last moment. His soul is willing to go, and he is the one who stops it.
Because the poem is presented as a version "from the Greek of Plato", it is a translation, and I can only judge the English. The idea of a soul that can slip loose from the body sits comfortably with the philosophical tradition Plato's name evokes. In Shelley's wording, though, the idea feels light, almost playful. The soul is not a grand abstraction. It is a "poor thing" that has "wandered thither", and the phrase makes it sound like a lost pet or a small child trailing after someone older. Nothing about it is rebellious or ambitious. It simply follows the kiss, and the kiss is cast as the guide, the one who knows the way. The speaker's mouth becomes a kind of trap on that route.
The construction makes the reversal feel more sudden than it is. The poem is essentially one long sentence that unrolls the event in order: the kiss, the soul arriving, the speaker holding it, the explanation, the verdict. The first person is slow to take charge. Early on we have only "my kiss, my soul", things that seem to happen to him. The "I" becomes an agent only in line three, "there I kept it", and again in the closing "cruel I". Both times he is the one who stops something moving. Line four then supplies the "For", the excuse that explains why the soul was there, and that explanation is what makes his act look so unkind.
The sound of the poem works in a similar way. The rhymes run ABCABC: "together" pairs with "thither", "beside it" with "guide it", "kept it" with "intercept it". The first pair is a near-rhyme, since the vowels differ, and the other two are exact. Every line ends on an unstressed syllable, so each one seems to trail off rather than land firmly. That gives the poem a soft, falling movement, like a breath let out, which suits a story about something drifting from one mouth toward another. The lines are short, roughly eight or nine syllables, and the rhythm is loose enough to sound like speech, a lover murmuring a confession. Rhyming "kept it" with "intercept it" ties the two halves of his guilt together, because the soul he possessed is now the soul he blocked.
Word choice delivers the final turn. For five lines the vocabulary is gentle: "together", "beside", "wandered", "guide". Then comes "intercept", a cold, almost official word, the sort used for stopping a message or a messenger. Set against "poor thing", it shows how the speaker sees himself, as a tender person who has just realised he has done something quietly harsh. The "Oh" and the exclamation mark keep it from being solemn. The reproach is half-serious and half-delighted, because the only way he can be guilty of stealing his own soul is if the kiss was powerful enough to dislodge it.
The poem's significance lies in how much it holds in so little space. It takes a familiar romantic exaggeration, that a kiss can draw out the soul, and turns it into a small moral drama about giving and withholding. It also shows what a translation can do. Whatever the Greek looked like, the English has become a self-contained miniature whose last line reframes everything before it. Read once, it is a pretty conceit. Read again, it asks whether a lover who clings to his own soul has loved fully at all.