You might think moths are just boring versions of butterflies, but they're way more than that in this story. No surprise there, though, since they're included in the freaking title—things in titles are usually pretty important. So let's take closer look at these fluttery creatures.
Moths first make an appearance as dried-up wings added to a salve for the narrator's bull hands:
Abuelita made a balm out of dried moth wings and Vicks and rubbed my hands, shaped them back to size and it was the strangest feeling. (3)
Right away, then, moths are associated with healing. And interestingly, their healing capacity is immediately connected to death—or so we're assuming, since the wings are dried-up and we can't really imagine anyone plucking wings from a living moth. Or a moth living long once its wings are plucked off. Life and death are brought close together here as the moth wings are used to help soothe the (very much alive) narrator's hands. And since Abuelita is involved, her impending death—and the transformation it inspires in the narrator—is subtly foreshadowed, too.
The most vivid appearance moths make in the story, though, is when they come pouring from Abuelita's mouth after she dies. Check it out:
Then the moths came. Small, gray ones that came from her soul and out through her mouth fluttering to light, circling the single dull light bulb of the bathroom. Dying is lonely and I wanted to go where the moths were […]; I wanted to rest my head on her chest with her stroking my hair, telling me about the moths that lay within the soul and slowly eat the spirit up. (16)
You know how the moths are used at the beginning of the story to heal what is broken? Well, now they show up as the narrator is cleaning her dead grandmother's body. We might think of Abuelita's body as permanently broken now, as beyond repair—and the moths' departure from within her as carrying her spirit away to safety. After all, the moths immediately go to the light and light often represents something good.
As an aside, we want to mention that the presence of the moths at the end takes this short story out of the realm of the ordinary, rocketing it into the land of magical realism.