Purling through the back loop (ptbl) is a way of twisting a purl stitch.
Instead of inserting the right-hand needle into the stitch from left to right and purling it, you insert it from right to left, similar to how you would to knit. However, unlike a knit stitch, where the yarn and needle tip are at the front of your work, to purl through the back loop, the yarn and needle tip are at the back of your work.
The following video demonstrates how to work a ptbl, beginning at 1:05. (The first minute shows how to knit through the back loop.)