The Star-Belly Sneetches have stars on their bellies. In Dr. Seuss’s story, the stars are green markings carried by one group of the yellow, bird-like Sneetches.
The star-bellied Sneetches use their markings as a badge of status and exclude Sneetches whose bellies are plain. The social division drives the first story in *The Sneetches and Other Stories*, published in 1961.
Sylvester McMonkey McBean exploits the rivalry with machines that add or remove stars. He initially charges the plain-bellied Sneetches three dollars to get stars, then charges the original star-bellied group ten dollars to remove theirs. His machines ultimately make the groups indistinguishable and leave the Sneetches penniless.
The story’s point is that an arbitrary visible difference cannot create genuine superiority. Readers sometimes remember only that the Sneetches are yellow or bird-like, but the defining detail in this question is the green star symbol—and the contrast between star-bellied and plain-bellied Sneetches.