Three short sentences carry the whole poem, and each one makes the picture worse. It opens with an idea, "Satiation dulls," then moves to an image, "Bloated faces," and ends on a physical act that is hard to look at: "Mouths expel what should pass elsewhere." The central claim is that having too much does not satisfy us. It numbs us, and eventually it turns us ugly and sick.
The first sentence runs against common sense. We usually think fullness is the happy end of wanting. Riley's verb, "dulls," says otherwise. A dull blade no longer cuts, and a dulled sense no longer picks out flavour or pleasure. The word "satiation" is abstract and a little clinical, a word from a textbook, and it is followed at once by a short, blunt verb. The sentence is only two words long and it stops dead. That abruptness acts out its meaning, because the sentence itself seems to lose its edge.
The second sentence drops the verb altogether. "Bloated faces" is a fragment, a snapshot with no action in it, and the missing verb makes the faces look stuck and swollen. The choice of "faces" over "bodies" or "people" matters. A face is where we expect expression, attention and individuality, and here it is simply puffed up. "Bloated" also has an unpleasant afterlife in the mind, since it can suggest something waterlogged or decaying. That is my inference, but the poem gives us so little that each word has to carry a lot. The plural, too, makes the faces anonymous. We are looking at a crowd, not a person, so the failing seems to belong to a whole group, not to one unlucky glutton.
The last line is the longest, and it is where the poem's restraint pays off. "Mouths expel what should pass elsewhere" avoids a blunt, crude word and uses a polite, almost medical phrasing instead. "Expel" is formal, and "elsewhere" is an evasive way of pointing at the body without naming it. The politeness makes the line more disturbing, because the image is plainly there but the language keeps looking away from it. The mouth is meant to be the point of entry, the place where things go in, and here its job is reversed. Anything that is taken in beyond what the body can use has nowhere left to go but back out the wrong way.
The line can also be read more widely, though the poem never says so. Mouths speak as well as eat, so "what should pass elsewhere" might also describe talk that is really waste, noise produced by people who have taken in more than they can use. The text supports the literal reading of overeating, but its brevity leaves this second reading open. The idea of excess could cover food, entertainment or opinion.
As for sound and structure, there is no rhyme and no regular metre. The poem is written in the plain rhythm of speech, and its lines grow from two words to two words to six. That slight lengthening feels like a build-up before the final, unpleasant image. There are small patterns of sound as well. The soft "s" in "satiation" returns in "faces" and again in "should pass elsewhere," so a hissing sound runs through the poem like a sigh of disgust. The heavy "d" and the "ll" in "dulls" have a thud of their own that fits the sense of something blunted. The title, which is only an asterisk mark, adds to the effect. With no title to guide us, we are dropped straight into the scene and have to decide for ourselves what we are looking at.
The poem's form also does the opposite of what it describes. It is a poem about bloat, and it is itself almost unbearably lean. It uses no ornament and no explanation, and it does not moralise. Instead of telling us that greed is bad, it shows us the result and trusts us to be repelled. A longer poem would risk becoming a sermon, but this one lets disgust do the arguing.
This compression is what makes the poem worth reading. In under a dozen words it gives us an idea, an image and a reversal of the body's natural order, and it leaves us to decide who the "bloated faces" are. They might be strangers, or they might be us.