Stitching in the ditch is a quilting term. It is a machine quilting technique that involves quilting in the seam line. This is commonly done with invisible thread (a little bit like fine fishing line), or you can use matching thread. This technique is usually chosen by quilters who do not like to see the quilting stitch. Because the stitch is "in the ditch of the seam line" it tends to disappear.
This technique can be difficult to master and miscalculating the seam can look unsightly. So care needs to be taken.
Stitching in the ditch is not often chosen by hand quilters. A more popular technique is to quilt 1/4" from the seam line which avoids extra bulk of the seam allowance. If you would like to hand quilt using the stitch in the ditch technique you can do so by quilting with a small running stitch. Do not use a back stitch.