A speaker is irritated by a moth, snapping at it to go away and leave them alone. The opening lines are short and curt, almost like swatting at the air. Then the mood turns. The speaker apologizes for trying to squash the insect, and that small admission opens something up.
After the apology, an image forms in the speaker's mind: the moth's face, "laughing, filled with hope." Of course, a moth's face can't really laugh, so the poem seems to be showing how the speaker's view has shifted. The pest becomes a small creature with its own feeling and its own eager reaching toward light.
The last lines step back into something larger and more reflective. Life begins in darkness, grows with light, and can't be pinned down by ideas. The mood moves from sharp and impatient to gentle, humbled, and quietly wondering, as if a minor annoyance has taught the speaker something about life itself.