It's a hairpin lace tool for crocheting. I bought this one, but I've since seen tutorials for making them at home, like this one.
The idea is to produce strands of crochet with loops either side, which are then joined together with crochet in various configurations to form straight or wavy patterns in doilies, scarves or shawls, I've even seen patterns for skirts and other garments.
It's quite meditative. You do one loop at a time and then flip the tool 180 degrees to do the next one, winding the yarn around it as you go.
This is a wool/soy/mohair blend yarn (Moda Vera 'Believe'), which is destined to become a decorative wrap when I finish (can't imagine it will keep me very warm, LOL). I've done one strip of 105 loops, and am nearly finished the second strip of 105 loops (that's 105 each side, so 210 stitches and flips in total) - only three more to go before I can start joining them up!
I have carpal tunnel in my left hand (I had the right hand operated on last year) and it is making knitting difficult at the moment, so this is a nice change of pace, as well as helping me practice crochet, since I am only a newbie and can't even manage granny squares which are actually square yet ;-) How is it that just the one hook can be harder to use than two needles? LOL.
I'll post pics when it's finished - provided I can manage the shell crochet border that is :-)