I'm sure someone else could explain this better than me - but I'll give it a try. After you sew a row together you stitch the seam allowance down by top stitching near where the fabric meets.
Here are two descriptions that I coppied and pasted from sewing sites:
"Topstitching is an extra line of stitching sewn on the right side of the fabric that parallels a seamline or is used to sew a hem. Topstitching is usually visible on a project, so it needs to look good."
Adding a top stitch gives a finished look to your seam and adds a bit of strength. The top stitch is sewn on top of a primary straight stitch, sewing through the top of the fabric and the seam allowances underneath.
"To sew the top stitch, lay open the previously sewn fabrics with right sides up and the seam allowance underneath. Fold the seam allowance (underneath) to the side where you intend to top stitch. You will sew through the top fabric and through both fabrics' seam allowance underneath to hold the seam allowances flat. Sew as you would a straight stitch, with the top stitch seam about 1/8 to 1/4 inch from the straight stitch seam."
I topstitch all kinds of seams - I like the way the fabric lays after. I do it on tiered tops, ruffles, patchwork skirts, etc. I think you can see it in the skirt photo below. The stitching above the pink ruffle - the top yolk is also topstitched above where the stripwork panel meets the yolk. Does that help any????