It's a head fitting standard Oral-B toothbrushes that is a sandpaper holder, so you can use your toothbrush as a small orbital sander, great for removing surface artifacts left by supports or bad overhang/bridging and some other model work. I made it because drills can be noisy and uncomfortable to work with small models and provide only rotational motion that can exert force on objects being sanded and cause to move making sanding less precise or damaging the surface.
Just cut a circle of sandpaper about 1.5 times the diameter of the head and cut eight or more radial incisions that go almost all the way to the head, then fold them inwards around the head and screw it into the shaft. Screwing in and unscrewing is easier when already mounted on the toothbrush. You could also put some microfibre cloth in in the same way to use for polishing.
I've taken the screw thread from the sanding stick by Mightynozzle: https://www.thingiverse.com/thing:2404850
generic PLA or ABS, same material for head and shaft to avoid different shrinkage