rookie question

It's a material clip, used to hold materials out of your way prior to completely making use of them. You may need to tie in a couple of items at the rear of the hook for future steps and this keeps them in place, out of your way. Another quick hint is if your tying in long hairs or feathers for a tail is to use a large spring snap clip mounted over the top of the vise head and lay the materials on there after they've been tied in place. This will keep them from falling to the side.