The speaker is out on a woodland walk in early spring, noticing small things one after another. Meltwater glimmers on top of the ice, a sign that winter is loosening its grip. Then the view narrows to a plastic trail marker, where a click beetle's antennae sweep back and forth, as if the insect is feeling out the way forward. The marker is a human mark on the landscape, and the beetle treats it as just another place to explore.
In the last stanza the scene deepens. Among ancient hemlocks, the speaker finds the patch of moss where they once spent a night. The poem doesn't say why or how that night went, so we're left to imagine it. The tone is quiet and attentive, with a hint of tender nostalgia. The title, "No Swimming," suggests a sign or warning, perhaps beside the icy water, though the poem never says so. Time moves in layers here: the thaw is passing, the trees are ancient, and one person's memory rests on the moss.