The ridge stitch pattern, or ridge stitch effect, is created by alternating rows of purl and knit stitches on a loom. In this video the ridge stitch pattern was completing by doing 2 rows of purl stitches, followed by 4 rows of knit stitches:
Row 1 & 2 Purl
Row 3 6 Knit
To continue the pattern you would knit as follows:
Row 7 & 8 Purl
Row 9 12 Knit
Row 13 & 14 Purl
Row 15 18 Knit