Mock ribbing imitates knit and purl ribbing by using needles out of work. The "Ladders" formed with the needles out work resemble purl stitches and create the appearance of ribbing (but not the same stretch).
Mock Ribbing (False Ribbing) is one of the first hem techniques that many machine knitters learn. It is basically a hem that imitates ribbing.
Mock Rib is quick and easy and you don't have to re-form any stitches. The hem pulls in and has stretch.