Basically, fake cereal, not (che)real. Chereal is most likely cheap, because it makes it easier to compete with the brand they actually copied.
These brands are also usually named something nearly the same, something that means the same but is basically just a cheaper version.
The brands also have an image that relates to the actual brand it is bootlegging, as well. Or something completely random. Like an image of three African American kids running after a kangaroo.
Some examples of chereal brands are Crispy Rice, Frosted Fruity, Oh's, and General Gnash.