Bonta's title, "Imponderable," means something too uncertain or vast to be weighed, and the poem tries to show that rather than explain it. It opens with two small images of settling: night sinking into a hole, and a fly landing on its supper. One is huge and quiet, the other tiny and a bit grubby, yet both have the same soft, unhurried drop. Then the poem says that both are "the same as asking a question / in a prayer."
That comparison is the surprising part. A question inside a prayer isn't really expecting a reply. It just settles into the dark and waits, the way night fills a hole or a fly finds food. The speaker seems to feel a mix of wonder and humility, with maybe a little dry humor in putting prayer next to a fly.
The mood is hushed and contemplative. Nothing is solved, and the poem seems content to leave the question hanging, which is exactly what "imponderable" suggests.