Correctly-formed crochet stitches all have components known as “loops”. There are front loops, and there are back loops. You can crochet into these loops in different patterns, a process that can be really interesting to experiment with.
Back loop single crochet stitch is one of the easiest examples of a stitch you can crochet using the back loops. There are many other stitches you can create as well. For more information, you may wish to view the following pages: