The speaker describes losing hope in a strange, quiet way: their hopes are being eaten by a moth that has no mouth. That image doesn't quite make sense, and that's the point. Something is destroying everything the speaker wanted, yet there is no visible cause, no jaws, no clear culprit. The loss is slow and soundless, like holes appearing in cloth that nobody notices until it's too late.
The second part turns hard. Guns, unlike the moth, are blunt and deadly, but the speaker says they "do not grieve / or think / to miss." The title plays on two meanings of "miss": to fail to hit a target, and to long for someone who is gone. A gun can miss, but it can never miss anyone. The poem seems to point toward violence and the emotional blankness of the thing that causes it, though it never says exactly what happened. The mood is bleak, cold and numb, and the short, bare lines leave the reader to feel the gap where grief should be.