Ben Wa Balls are balls that can be inserted vaginally and kept in for short periods of time to strengthen vaginal muscles. (Make it easier to clench, generally tighter, etc)
I have a pair, but I honestly forget to wear them often. When I do wear them, they provide a really faint, but nice vibrating sensation due to having loose weights inside, which move when I do. Although I haven't noticed myself any tighter after use, they do get me quite wet, surprisingly.
I don't regret buying mind, but I don't really use them much either. XD