Knitting through the back loop (ktbl) is a way of twisting a knit stitch.
Instead of inserting the right-hand needle into the stitch from right to left and knitting it, you insert it from left to right, similar to how you would to purl. However, unlike a purl stitch, where the yarn and needle tip are at the front of your work, to knit through the back loop, the yarn and needle tip are at the back of your work.
The following video demonstrates how to work a ktbl. Both English and Continental styles are shown in the first half of the video. (At 1:05, it switches to purling through the back loop.)