A cover stitch is sewn without the knife on the serger. It is commonly used to hem tee shirts and stretchy fabric. Two lines of straight stitching are on the right side and looping threads on the wrong side which allows the fabric to stretch without bursting the stitches.
Triple Cover Stitch
This is a triple cover stitch sample.
Triple Cover Stitch
Debbie Colgrove, Licensed to About.com