Unless otherwise indicated in a pattern, an understitch is a normal-length (2-3) straight stitch across the layers of fabric underneath the exterior (outward-facing layer) that is fixed at both ends. Like the Topstitch, you want to try to get the stitch as close to the original seam as possible, but because it is invisible, it is not as critically important that the stitch be straight.
Often, you will do an understitch after you trim the seam and clip the curves (trimming down the seam allowance on a bias and creating notches at the curve for ease--article on its way). If you iron your folded seam before you stitch, rolling the seam back just slightly so that it's along the inside edge of the underside of your garment, it will make the seam invisible to the outside, and then understitching will help keep that structure in place.
To practice the understitch, use a leftover piece from practicing a Simple Seam. Iron the seam allowances to one side, and then fold the top layer to the side, and stitch the three exposed layers together (the two seam allowances, and then the other piece at the bottom).