The speaker describes a recurring nightmare: a crowd of open mouths with rotting yellow teeth, sore gums and blistered tongues, creeping steadily closer. The images are deliberately disgusting, and they work on more than one sense. We don't just see the mouths, we smell the hot, musty breath, and the speaker says it overwhelms them.
The form does a lot of the work. When the mouths approach, the words collapse into long unbroken strings with no spaces, so the reader feels the same crowding and loss of control. Then "closer / and / closer" is stretched down the page, slowing everything into dread. The speaker always wakes just before the mouths arrive, but that doesn't bring relief. The final line runs together again, because the sensation of tongues crawling over the skin lingers after the dream ends. The mood is claustrophobic and queasy, and it leaves the feeling that the fear isn't really over when the speaker wakes up.