A Floss Threader is a plastic needle-like accessory that you attach to a length of dental floss to help snake it under the arch wire. It enables you to floss correctly when you have braces on your teeth. Most Floss Threaders look like thin dull flexible plastic darning needles.
A similar product, Threader Floss, is a length of floss that has a stiff end. It enables you to floss between your braces without attaching a plastic Floss Threader needle.