Floss holders are devices that allow people to clean the surfaces of teeth that face other teeth and are next to areas without teeth that are not accessible with a toothbrush. Because these areas do not build plaque as much as the other three it is recommended to floss at least once a day.
The proper way to allow floss to clean these surfaces and to put it in between the teeth and have it form a C-shape and scrape one tooth, then press it against the opposing tooth and scrape the other tooth.
Things to consider for floss holders are ease of use, cost, and if it is disposable. Fully disposable floss holder add to the landfill and are more expensive long term but are easier to use.