SID is short for Stitch-in-the-Ditch. The “ditch”, in quilting terminology, is the seam line between two patches on the right side of your quilt top. When quilting the top, stitching in the ditch will make your patches stand out and give crisp and clean lines to the finished quilt.
When SIDing, the object is to have your quilting be as invisible as possible. Since seam allowances are generally pressed to one side, you sew on the ‘low’ side (the one the seam allowance is NOT lying under), as close to the seam line as you can. When the fabric relaxes after the quilting process, those stitches will lie somewhat underneath the ‘high’ fabric and hardly been seen.