Stitching in the ditch is a machine quilting technique. Seam lines (the “ditch”) on the quilt top are stitched while using a walking foot on your machine. A quilt top can be completely done with “stitch in the ditch” quilting or may be quilted with other techniques.
Stitching in the ditch does save time because you do not have to mark your quilt top with a design.
In this photo, I was stitching in the ditch around the patchwork using light pink thread. In the middle background white triangle on the right, you can see where I made a mistake and “jumped” out of the ditch. If I would have used invisible thread this might not have been so noticeable.