BenWa balls are used to strengthen and stimulate the inside walls of the vagina. Women usually insert them in the vagina and try to keep them inside as long as possible. The vibration and movement of the benwa balls should help stimulate the vagina and arouse you.
BenWa balls will also help tighten the vaginal muscles. You may want to start out with the bigger ones, and work yourself up the smaller benwa balls.