Bobbin work is a decorative technique that uses thicker or specialty threads. Often you have to wind the bobbin by hand. You often need to loosen the tension on your bobbin case, which is very scary for most people, so the alternative is to get a separate bobbin case for this type of work. Since the specialty thread is in the bobbin, you work from the reverse side of the fabric, with right side down. You can free motion, or use specialty stitches.