Het is short for heterozygous. Usually it's used when referring to albinos but it applies to any recessive genetic mutation. It measn the animal carries an allele for the gene but not both so it doesn't show (e.g. het albinos look like normal snakes but can make albinos if bred to albinos or other hets).
Jags are a Carpet python morph that look very different to normal. Conversations about jags usually turn nasty here because even though they are Australian snakes the original ones were smuggled here illegally from overseas