This video shows how to knit a stitch through the back loop. It's the way you can twist your knit and tight the fabric by stitching through the back loop. People usually do it through front but it does not twist your stitch as much as through back loop and regular stitch does not tight the fabric as much as through the back loop. When you knit a stitch through the back loop, it sits differently from the regular stitching. So first you need to stitch through the back loop and do your next stitch as you always do. it will make your fabric tighter and give it a little bit different look.
Have an iPhone? Check out all 200+ new features coming in iOS 13.