The speaker sits in a garden on a cold day and lets small observations drift into one another: two seats filled with leaves and twigs, a strange spam comment about naked people, a train whistle. The whistle pulls up a childhood memory of a beast in recurring dreams, which the speaker later understands as a simple matter of standing in the train's way. The fix was to lie flat and let it pass overhead.
That idea of yielding runs through the whole poem. A shed snake skin and a numb fly in the cold suggest that change and survival are hard, even when necessary. The nudists become a playful image of people who have mastered stepping aside, so open that their thoughts and digestion show. The mood is quiet, wry and gently reflective. The speaker seems to find calm in accepting that nature, like the wind, sets its own terms, and we do best to adapt.