Topstitching is where you sew on the right side (or the top) of your sewing project. It can be used as a functional and/or a decorative stitch that is usually sewn 1/4″ away from the edge of a seam or fold.
As the stitching is visible on the right side of your project you can choose a matching thread colour that will make it “invisible” such that it blends in to the fabric, or choose a contrasting thread that will result in a “decorative” stitch that stands out. One example of a decorative topstitch is the gold thread used on jeans.
Topstitching can also be used for construction e.g. securing a patch pocket to a shirt. It can also be used to hold seam allowances and facings in place so they don’t roll to the right side for example around the neckline.