Het is used when talking about genetics, and usually about recessives like Pied, Clown, Albino, Caramel Albino, Genetic Stripe, Axanthic to name a few.

When you breed a visual of any of the ones above to a normal - all the resulting offspring are going to look like normals. But all of them will carry one copy of the gene, and be considered 100% het for .... whatever the visual was.

In order to produce, say a piebald, you could breed two 100% het piebalds and produce a visual piebald, if both of the hets passed on one copy of the gene.

Here's a good visual explaination: Genetics

Co-doms like pastels are considered the het of their super form. So a pastel would be het for super pastel.

A Yellow belly is het for Ivory

and so on!