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It means that the horse is homozygous for NOT carrying a cream gene.

Dun is a binary gene--there is a dominant form and a recessive form. One copy of the dominant form completely overrides the recessive and gives you a fully dun colored horse. Two copies of the dominant form does not give you a "more dun" horse than one copy. Thus, dominant dun (in the game at least) is D and recessive dun (no dun dilution) is d.

Cream is an incompletely dominant gene. Mixing the various allele possibilities gives you different phenotypes (different physical manifestations). So, you have no dilution, C, which doesn't affect the base color of the horse. You have the cream dilution, Ccr, which when heterozygous lightens the coat color variably (depending on base color) and has a variable effect on mane and tail color and when homozygous turns the entire horse a creamy whitish color and affects eye color. And you have pearl dilution, Cprl, which when heterozygous may cause slight lightening of the coat color (I really notice it with wild bay) and when homozygous gives a champagne like dilution and a pearly sheen to the coat. If you have a horse that carries both cream and pearl, the coat is pinkish and has the sheen. Because all three alleles affect coat color in roughly equal amounts, all three are capitalized.

Agouti is another interesting one, less incompletely dominant than variable dominance at the same locus, with wild bay more dominant than regular bay. But brown is incompletely dominant because a brown horse with one At is a different color than one with 2.... it's enough to make your head spin!

Genetics are the same for all animals, just the specifics of expression change. Dogs, cats, cattle, goats, horses all have MITF and KIT as major pattern genes. All have black/red switches and agouti patterns. They might get called slightly different things and might express slightly different but they are the same things.