"Het" refers to heterozygous.
It is a word used when talking about recessive genes and traits.
So if it is a Het for Leucistic (which I think is used to loosely, that is the leucistic part) than that means when that normal colored dragon is bred to another het for luecistic normal colored baby, then you will get 25% of the offspring to be leucistic and the rest will be Hets. But seeing as leucistics on the markets are not really luecustics just super hypo animals (yes even Dragon Dens!)than what you really will get is some a couple nice white babies.
If a het is bred to a "leucistic" animal then you get 75 percent to be white and the rest are hets.
The reason I say there are not any (or not many at least) true leucistic beardies is because none of them are white, and stay white, they all seem to be off white, or some times turn light brown. Only when totally fired up are they totally white. If that is what you are looking for then great. Just dont be fooled.