Most cotton is crafted from single yarns, meaning that strands of the cotton staple are spun to make a single thread. This thread is used to weave traditional shirting fabrics, which are not quite as luxurious as the ones made from double-cotton.
To make a two-fold yarn, a long and fine staple yarn is chosen first. Then, these separate yarns from the cotton staple are spun together again creating a very fine yarn that has essentially been folded together twice. This is finally woven into a shirting which has a smoother, silkier finish.
The extra care and processing that goes into manufacturing two-fold cotton is reflected in the higher price point of the final piece.