what is an understich?

Question

and what does it mean

Answer 1

"Understitch" refers to a stitch that is put in to keep the garment or fabric from rolling. For example, if you are sewing a facing or a lining into a garment, you'll need to secure that fabric so that it won't move while you sew in a more stable stitch. You would stitch, with a simple needle and thread, the facing to the garment. Then you could grade and clip the seam allowances. The facing will be held firmly in place until the final seam is sewn.