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.
If you’d like to learn how to do the back loop single crochet stitch, or try an easy project using this stitch, I recommend this easy ribbed crochet baby beanie pattern for beginners. It’s a quick project that will get you plenty of practice at crocheting through the back loops — plus you’ll end up with a useful project that you can give as a baby shower gift or keep for a little one in your own family.