This stitch is the same as the front loop only stitch, except (as the name implies) we are crocheting into the back loop this time – the one furtherest away from you.
For a regular stitch, you would normally insert your hook under both loops. Instead, insert your hook into the back loop only, and then complete your stitch like you normally would.
Crocheting in the back loop only gives a slightly visible seam, and is useful for making flat bases. Such as those at the top of a top hat, or the base of a foot.