Probably the best way to understand what these are and how to use them is to look at the photographs.
These are nice if you are sewing your complete project by hand (not everyone has a sewing machine :-).
Basically instead of sewing a channel to put a drawstring through, you slide the clip's legs over the top edge (hemmed or not as you prefer) and stitch it in place through the pre punched holes in the legs. After that put a cord through them, and viola! done :-)
The directions are entirely in Japanese with some drawing.
There are plastic, 8 in a pack, and are 1 inch long, and the hole is 1/4" wide.
Open since 2005, but currently online only, A Threaded Needle is located in British Columbia, Canada. I am always happy to hear from you by e-mail firstname.lastname@example.org