Topstitching is sewing that is visible from the outside of the garment. Topstitching can be very close to seam lines to hold two or more pieces together or to get a seam to lay flat. It may be far away from seam lines, as in the case of hems. Many times, it has no functional purpose other than decoration.
Most shirt collars are topstitched because there are two layers of fabric and one layer of interfacing. The topstitching secures all the layers together and sharpens the seam edge on the collar. You can barely see the topstitching in the image above because thread was used that matches the fabric.
Sewing machine top stitches can be incredibly small. For example, a well made men's shirt can use stitches that are 24 - 30 per inch (25.4 mm).
If you are applying a topstitch intended to add to the design, you may want to use a larger thread, contrasting color or larger stitch to make it really stand out.
Many sewing machines come with decorative stitches that you can add as a topstitch.