It's a tatting shuttle by the look of it. Tatting is an intricate way of making knots in thread so that it looks like lace. If you want to learn it, get someone to demonstrate it for you as initially there is a knack to the knot being jerked slightly so that it works properly and it is a little difficult to pick up from a book. I learnt from a book and did not get it until I jerked it in frustration - it is more difficult to unpick mistakes than it is with crochet or knitting.