Shelley opens with a wish so plain it could be spoken across a room: "O Mary dear, that you were here." There is no argument and no grand philosophy, only someone in "this azure Italy" wishing another person were beside him. The title suggests, though the text never says so, that Mary is his wife, Mary Shelley. What the poem makes certain is the distance between speaker and listener, and its central idea follows from that distance. Absence changes how a place looks. The Italian sky is lovely, but its loveliness only points to what is missing.
The form suits this simplicity. The lines are short, mostly around seven or eight syllables, and they bounce along like a song. The opening couplet, "here" and "clear," sets a lullaby-like tone. Then the rhyme scheme folds in on itself: "bird" and "heard" enclose "mate" and "disconsolate." Later the pattern tightens into alternating rhymes, "soon" with "moon" and "far" with "star." The last line of that passage, "Thou, beloved, art to me," has no partner nearby, so it hangs in the air as the sentence's emotional point. The slightly uneven line lengths make the poem feel improvised, as if the feeling arrived before the meter could tidy it.
The central image is a bird, and it works in a more complicated way than it first seems to. Mary's voice is "like a bird / Singing love to its lone mate / In the ivy bower disconsolate." The comparison is meant to flatter her, yet the picture it paints is of loneliness. The word "disconsolate" properly belongs to a person, but here it is given to the bower, so the whole scene seems to grieve. Judging by the imagery, the speaker has slipped his own sadness into the comparison. He may be the "lone mate" waiting for the song.
A smaller change in language does a lot of work. At the start he says "you," which is conversational and close. By the middle he says "thou art far," which is more devotional. The plainest sentence in the poem comes right before the poetry swells: "I am not well whilst thou art far." It is blunt, almost like a note sent to explain an illness, and the lofty similes that follow gain force from being set against it. "As sunset to the sphered moon, / As twilight to the western star" is deliberately unfinished. We are never told what Mary is to him, only that she stands to him as these pairs stand to each other. One reading is that the moon shines with borrowed light and the star appears only as day fades, so she is the condition of his shining at all. That is an interpretation rather than something the poem states, but the open comparison invites it.
The text as we have it is also full of holes: "your brow more... / Than the ... sky." Whatever the reason for the gaps, they have a strange effect. The comparison is about her beauty, and the one detail we cannot read is the one that would complete it. The poem enacts the very absence it describes, and the reader is left, like the speaker, reaching for something not there.
The ending is the best touch. "The Castle echo whispers 'Here!'" can be read two ways. The landscape might be answering him, as if Mary were present after all. Or the echo might only hand his own word back to him, which would make her absence louder. The poem's last rhyme is the word "here" rhyming with itself, so the echo is built into the sound. He calls out a wish and the world replies with the wish's own last word, the only answer he gets.
The poem is significant for its smallness. Shelley is a major Romantic poet, and this is a private, unguarded voice. Even unfinished, it shows how little ornament a feeling needs. A few rhymes, one lonely bird and an echo are enough to make a reader feel the empty space beside him.