Topstitch is a line of machine sewing that is used to add a decorative finish and also to attach pockets or other items to your projects.

1). When topstitching for decorative purposes, simply sew a line of normal stitches close to the seam on the right side of your project, as shown. I recommend sewing a distance of 3–5mm (1⁄8–1⁄4in) away from the seam.

Topstitch 1

2). When topstitching as a method to attach an element, such as a pocket, to your design simply pin or tack (baste) the item right side up on top of your project (also right side up). Sew the item in place with a line of stitching close to the edge of the item being attached, as shown. I recommend sewing approximately 3mm (1⁄8in) away from the edge.

Topstitch 2

Source :
Sew Cute to Carry
12 Stylish bag patterns for handbags, purses & totes
Melanie Mcneice