Reverse Single Crochet aka Crab Stitch
It is meant for borders and final trim work. You can see it on the heart as it has bumping rope appearance. It’s a great stitch to be honest with you.
So reverse means just that. You need to go back in the direction from which you came. So instead of advancing forward like regular single crochet, you go back to the previous single and do a single crochet. It naturally creates the bumps that you can see.
Give it a try, tutorial is provided below.