The ptbl (meaning "Purl through the back loop", also called "twisted purl stitch") produces a twisted purl stitch. It is formed identically to the purl stitch except for one key difference — the angle at which the needle is inserted into the stitch to be worked.
In a normal purl stitch, we insert our needle from right to left through the front leg of the stitch:
In a pbtl, we insert our needle from left to right through the back leg.
The resulting stitches are twisted on the needle.
Check out the post on the reverse stockinette stitch.