Two short couplets, and a strange idea at the center: absence itself has a body, a swan, and that swan has died. The poem says it died "having too much world," as if even nothingness can be overwhelmed by existence. That is a odd, quietly funny thought, and also a sad one.
The second couplet turns personal. The speaker says "I miss" the swan's habit of "absenting himself," which means they miss the very thing that used to hurt: a person or presence that was always leaving, always not quite there. Now that absence has died, there is no more leaving to miss, and the speaker is left with a double loss. The phrase "his place full of down" adds a soft, physical image of feathers and an empty nest or bed, still warm-looking but vacant.
The mood is tender, puzzled, and gently paradoxical. Grief here doesn't make a big noise. It sounds like someone turning a riddle over in their hands, finding it still hurts.