I think it means this:

Het = An abbreviation for heterozygous.

Heterozygous = Possessing two different genes for a given trait. An animal with one mutated, recessive gene still appears normal; its mutated gene can be inherited by future offspring. A co-dominant animal is heterozygous for the dominant form of its mutated gene, yet is different in appearance than both the wild-type and homozygous forms.

found this: Basic Genetic Reptile Vocabulary.