The double moss stitch is a simple stitch pattern, that consists of only knit and purl stitches. It consists of 4 rows that get repeated over and over.
Row 1: Knit 1, purl 1 - repeat until the end of the row.
Row 2: repeat row 1.
Row 3: Purl 1, knit 1 - repeat until the end of the row.
Row 4: repeat row 3.
I think this pattern shows up particularly well with rustic yarn, and with high twist yarns, that show the contrast between the knit and purl stitches quite well.