Take a look at the in-seams of your jeans (and possible the out-seam). Those two parallel top-stitched lines are holding a fell seam in place. A felled seam is incredibly popular seam; it neatly hides raw edges whilst reducing bulk.
You can use the felling foot edge to align the raw edges of your fabric, creating the perfect distance between each stitching line.
A felled seam can add a decorative touch when sewn in a contrasting embroidery thread, such as blue jeans and golden thread.
A common sized felling foot is 9mm. This size refers to the distance between each needle, thus the space between each stitching line.