If something is sewn, it's made or repaired with stitches. A sewn patch, for example, is attached to the knee of your jeans with stitched thread.
You can embroider a sewn design on fabric, or a use a needle and thread on a sewn repair to your favorite pair of socks. Anything sewn has been fastened, patched, or stitched. The adjective sewn is also the past participle of the verb sew, which is rooted in the Old English siwian, "to stitch, mend, patch, or knit together."