Topstitching

Topstitching is any line of stitching sewn from the right side of the fabric, visible as a finished design element rather than hidden inside a seam. It usually runs parallel to a seamline or a finished edge - collar edges, hems, waistbands, the outer leg seam of jeans - anywhere from an eighth of an inch to half an inch away from the edge it's reinforcing.

It does two jobs depending on where it's used, sometimes at once: holding facings, seam allowances, or a folded hem flat against the garment so they don't roll or flip to the outside, and adding visible durability or decoration - the flat-felled seam relies on it structurally, sewn through every layer, while a lot of decorative topstitching in contrast thread is doing almost nothing but showing off the seam underneath it.

A longer stitch length than your regular construction stitching (I go up a full step or two) and a slightly heavier or contrast thread both help topstitching actually read as an intentional line rather than disappearing into the fabric or looking like a mistake - it's meant to be seen, so sew it like it's meant to be seen.