Piping is a narrow strip of fabric (often with a cord inside for volume) sewn into a seam so that a thin, neat rib runs along the line. Piping plays a double role: it emphasises and 'draws' the lines of the cut (collar, lapels, pockets, seams), adding graphic clarity to a piece, and also strengthens and neatly finishes the edge. Piping in a contrasting colour is an expressive decorative device that gives clothing a smart, 'uniform' look (recall the piping on military tunics or sports jackets). Piping in the same colour works more subtly, merely structuring the form. It is a small detail that betrays the level of tailoring and the neatness of the finish.
