I had no idea but your question got me intrigued. According to teachsew.com it is this:
"Triple Stitch
Another great stretch stitch is the “Triple Stitch.” This stitch is typically depicted on your sewing machine by three horizontal lines drawn closely together. This stitch stretches because it takes two stitches forward and then one stitch back. You may need to try longer stitch lengths for best results."
I think it relates mainly to sewing knit fabric.
I have some patterns for appliqued rag quilts but all you do is stitch the applique using a regular stitch, quarter or half an inch in from the edge, depending on what look you want, then clipping the seams as per usual. I am guessing this triple stitch just creates a stronger seam.
Your pattern seems to read that you sew the applique in the traditional manner then "create" the raggedy border by sewing a strip round it...seems like a bit of a waste of time to me, but maybe the purists would disagree.
Thanks for the interesting thread....as usual by dropping by this site, I learned something new today.