The back stitch is a form of hand sewing in which the stitches go in a backward motion.  This is done by starting with the needle on the underside of your fabric, bringing the threaded needle up through the fabric and going back down behind the point where you started.  The back stitch is a handy stitch to know and is used in embroidery, especially when one needs fine lines, and also good in outlining areas, such as in appliqueing.