Het means that a snake carries one gene for a particular trait. It stands for heterozygous. Homozygous means that it has two genes for that particular trait. Recessive traits (example: albino) require two genes for the trait to show up, while dominant traits (such as normal type) require only one. Other traits may be co-dominant or incompletely dominant (such as pastel), which essentially means that a snake with one of the genes looks different from a normal type and a snake with two genes for that trait looks different from both normal and the heterozygous form.

You can be certain that offspring are heterozygous if one parent is known to carry two of that gene and the other has none, for example breeding an albino to a normal. All of those offspring will be 100% certain to be heterozygous for albinism. If two of those offspring are bred together, then you have a 25% chance of producing an albino, a 25% chance of producing a normal, and a 50% chance of producing hets. Because the albinos are visibly different but the normals and hets can't be told apart, all the ones that look normal are referred to as 66% possible hets for albino (because 2/3 of normal looking are statistically het, 2/3=66%). If you bred a 100% het to a normal, the resulting offspring have a 50% chance of being het and so are referred to that way. A "possible het" can be upgraded to a 100% het if it breeds with another animal (either a het or a homozygous recessive) and produces a homozygous recessive offspring, because then it's been proven.

Hope that helps!