The speaker is describing the closeness of holding someone and feeling their heartbeat. They reach "deep into the layers" of the other person, and the language is physical and tender: heartstrings, vulnerable skin, fingers barely holding that "ba-thump-ba-thump." The heart becomes a small ember, something warm and precious cupped in the palm.
As the poem goes on, the speaker stops simply listening and starts merging. Their own "melody" fits into the other person's rhythm. The musical terms, "syncopation," "canon," "soli," suggest two separate parts that overlap and echo each other without losing their own identity. The odd weather, with its tricoloured rain and sunlight tearing the sky, adds a dreamlike, almost magical feel.
The mood is intimate, hushed and quietly joyful. The ending shifts from "only i can hear" to "only we can hear," which suggests that the private happiness of one person has become something shared. Love here is a song that belongs only to the two of them.