Hypo is a recessive trait which means it needs to be homozygous to be visual i.e. it needs to carry 2 of the same recessive gene traits... A het carries one of those traits. Somebody who knows biology and stuff could probably explain it better in a more scientific sense.
So a spider HET hypo carries one non visual hypo trait. It means that if you breed it to a visual or another HET you can produce visual hypos.