Accessory for replacable toothbrushes, that clear plaque between the teeth. It is a low waste solution, because only the head gets thrown out, and you can keep the handle.
Do you need this?
If your gums are perfectly healthy, then dental floss is a better solution, while brush is easier to use. But if you have gum recession or the interdentals are opened, then this brush is more effective then dental floss.
Proper use should be learned at a dentist.