Few poems say so much in so few words. The speaker looks at death and, surprisingly, calls it a miracle rather than an ending. The idea is that death is a kind of making: inside the body, which is never really "finished," it produces "most rare things." The title points the same way. "(De)composition" holds both decay and composition, as if breaking down and building up were one process, and the brackets let the reader see both words at once.
The second stanza turns into a question: how could death be "locked up / between two hopes"? The speaker seems to resist any neat framing, perhaps the hope that comes before death and the hope placed after it. Judging by the wording, death won't stay inside those boundaries.
The mood is quiet, curious, and a little awed. There's no grief on the surface, only wonder and a gentle challenge to how we usually think about dying.