RST is an acronym for Right Sides Together. This is a common notation in pattern instructions which means that when you sew 2 pieces of fabric together you do so with the right sides together. The right side of the fabric is that which has the pattern on or has the correct texture. It is the prettier side of the fabric that you want to show off on the outside of your garment. Some fabrics do have the pattern showing on the wrong side but it tends to be duller than that on the front.