You put them in your pussy and wear them in secret so they stimulate you as you walk and more and, jiggle. They are also known as Ben Wa balls.
Most of them have a small steel ball INSIDE the balls themselves so they silently roll around inside you as you move.
They keep you stimulated from the inside, and also help strengthen your pelvic/kegel muscles.
You can get single ones but most of them are double to fill you up better and press on your g-spot