If you've ever had to put a stitch (or several) back on your knitting needle either because they slipped off the needle or because you were fixing a dropped stitch, you might have wondered how the stitch was supposed to go back on the needle.
Stitch orientation is important because if you put the stitch back on the needle incorrectly, when you next work the stitch you may twist it.
A stitch in knitting is usually thought of as a loop, but when thinking about stitch orientation you can also say that a stitch has two "legs" that straddle the needle. Typically the right leg should be at the front of the needle when you knit (or purl) a stitch, while the left leg should be at the back of the needle.
I write "typically" because there is a style of knitting, known as combination knitting, in which the purl stitches are worked in a way that makes the knit stitches present with the left leg in front.
Whether you knit combination or just have a misplaced stitch on your needle, should you find a stitch on your needle is twisted in this way, you have two options.
If the placement was an isolated accident, you can carefully slip one stitch at a time off the needle and turn it the proper way before putting the stitch back on the left-hand needle and knitting or purling as usual.
If you knit combination, somehow have a whole series of stitches with the "wrong" leg forward or just don't want to bother with rearranging the stitches by hand, you can simply knit the stitches through the back loop. In this case the back loop is what would usually be the front loop, so the stitch will be "twisted" into the proper orientation.