100% het means that the animal carries one copy of the recessive gene. It was produced by breeding a visual recessive to a normal (or a co-dom).

50% het means that the animal has a 50% chance of carrying the recessive gene. It was produced by breeding a 100% het animal to a normal (or co-dom). It has to be bred to prove it out. Since the 100% het has one copy of the recessive gene and one copy of normal, it's a 50/50 chance that it passed on either a copy of the recessive gene or a copy of the normal gene.

66% het means that the animal has a 66% chance of carrying the recessive gene. It was produced by breeding two 100% hets together. In a four egg clutch, statistically, 1 in 4 will produce a visual animal. Out of the other 3 normal appearing babies, 2 out of the 3 statistically have a chance of inheriting the gene, or 66% of the normal appearing babies have a chance of inheriting a copy of the gene. It has to be bred to prove it out.