"het" means heterozygous, meaning the animal carries one recessive gene for a trait. Let's use albinism as an example. The animal has to have both genes for albinism to appear albino (we'll call it "aa"). A het would be "Aa" meaning it has the recessive albino gene, but the dominant gene overpowers it. "AA" would then have both normal genes. If you breed two hets, you should end up with approximately 25% normal "AA", 50% hets "Aa", and 25% that display the trait "aa" (obviously the actual reproduction doesn't follow this pattern exactly though). I don't know if you needed that much info, but I figured I'd give it just in case.