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