A thread chain is a string of loops created by thread. Almost anyone who has crocheted has played with making a chain of yarn or sometimes it's called finger crocheting. The same technique is done with thread to create a thread chain. One of the ways that a thread chain is used is to anchor a lining. Many linings are totally separate from the garment at the hem area and may twist from the desired position. A thread chain to anchor the lining to the body of the garment, will allow the lining to remain free but prevent it from twisting.
A thread chain is also a secure way to hang a homemade Christmas ornament.
I sew thread chains on the ends of my curtain tiebacks for an easy way to secure them on a wall hook.