that is stitch in the ditch which means to stitch within the seam line or right next to it. i do next to it. you simply put your hands on either side of the seam going under the needle and stitch very close to the seam line. when you let go of the fabric the stitching won't show for the most part.
i don't like stitching on top of the seam threads myself. but it's an easy way to get a quilt finished.