A speaker hears hammering on a September afternoon and follows the sound backward, one "on" at a time: nails into planks, planks onto studs, studs onto beams, all the way down to a foundation, a hole in the ground, a clearing, and finally the hollow itself. The list works like the building it describes, stacking piece upon piece until it reaches the ground. Then it adds a layer of history. The hollow was lumbered two hundred years ago to make charcoal, which was used to forge iron nails. So the nails being driven today have a distant ancestor in the same place.
The last two lines turn the scene into an image. The speaker says the hammering reminds him of the sound the sky makes when it tries to bolt itself to the earth. The mood is quiet and reflective, with a little wonder in it. Ordinary construction becomes something larger, as if people have always been trying to fasten heaven and ground together.