All "heterozygous" means is that two different forms of an allele exist in the same location on two different homologous chromosomes. Heterozygous animals will phenotypically express whatever is encoded by the dominant allele. Obviously, if one of the alleles codes for a recessive trait, it will not be expressed in a heterozygous animal.

And before anyone else brings this up about the lettering...Technically the small letters are used to represent recessive traits, and the cap letters are used to represent dominant traits. But, if you're into Punnett squares, do whatever is comfy.