It's called a Double Chain Stitch. If you put those words into youtube, you'll find videos showing you how to do it. Ira Rott has a video where she demonstrates how to do it.
This crochet double chain stitch is an alternative for the crochet icord and or the chain less foundation chain..... there are many vids on you tube on how to knit and or crochet the icord.
Thank you for posting the picture. I had to try it immediately and the result is really plesing. Will use it for future projects as bags, baby booties ...