The poem is about the struggle to hold back what you want to say. The speaker asks how anyone learns to "swallow your tongue", meaning to stay silent, when there are "ghosts between your teeth." Those ghosts could be old fears, memories, or things left unsaid that cling to the mouth and refuse to go away.
The epigraph, "I eat monsters in the dark," sets up a tough, defiant voice, someone used to facing frightening things alone. But the poem itself sounds more tired and uneasy than triumphant. The speaker seems to be dealing with something inside that is hard to contain, and the last lines, "Your mouth is a horror / My friend," turn the focus around. It could be the speaker talking to themselves, or to someone else. The tone is both tender and unsettling, since "my friend" softens an accusation. The mood is dark and intimate, with silence and speech both feeling dangerous.