Three short lines capture a farewell at the moment it happens. The speaker is seeing someone off, and the poem seems to place us right at the kiss, the brief touch that closes the parting. The title, "Such sweet sorrow," echoes the famous line from Shakespeare's Romeo and Juliet, and it names the mixed feeling at the heart of the scene: tenderness and sadness held together.
The most striking image is the "bone moon." Bone suggests something pale, hard, and bare, so the moonlight feels cold and a little ghostly rather than romantic. The light seems to fall "over the kiss," as if the moon were watching, or as if the kiss were being drained of warmth even as it happens. Judging by the imagery, the speaker feels the ache of separation under the surface of the affection.
The mood is quiet, hushed, and wistful. There is no explanation of why she is leaving, which leaves the moment open and a little haunting.