Back stitching is done by sewing backward and forward at the beginning and end of a seam, on top of the seam stitches to prevent the stitching from coming undone. If you are sewing with a very fine fabric you may not like how back stitching leaves a relatively large amount of thread that can show through fine fabric, or change the way a fabric will drape or hang. In those cases you want to use a lock stitch.
In some cases it is best to sew off the fabric, leaving a long tail of thread and then knotting the thread. Sewing a dart is an example of using this technique. Back stitching or a lock stitch would leave an unsightly lump at the pointed end of a dart, but sewing off the end of the point allows for a smooth transition. What is a Dart and How to Sew Darts