Het is short for heterozygous which is a term for recessive traits such as albinos. In recessive traits both parents must carry the gene for the trait to be visible. A het animal looks normal but carries a gene for that trait, When an albino is bred to a normal(in Ball pythons for example) the babies look normal but are 100% het for albino. If you breed that het to another 100% het their babies would only be 66% het meaning the possibility of them carrying the albino gene is 66%.

Hope this helps