They're cymbals. You ding them together. I played The Dalai Lama in a panto before Christmas and these did the trick (If he used them I am sure he would probably have some of these as well). Build quality is as you would expect (two pieces of machined metal... can't go wrong really) although after 7 shows the elastic bit was starting to look a bit tired... Not bad value for money and pretty much what you would expect really... no surprises!
I always wanted one of these, and - though there are cheaper ones on amazon - this was the only one that shipped to Scandinavia. For 3 pounds they seemed like a fun thing to play with, and fit aesthetically into my small make shift Hindu shrine. They are not, however, the kind used for rituals or ceremonies, but the kind used by belly dancers or just as instruments.
Like the first reviewer, the sound is not optimal, but not awful either. For the price, the sound is ok and unless you are somehow using them professionally, "they'll do". One small point: The image on the product details is a little deceiving as the straps that you use to hold the zills are not tastefully brown or bronze, but brightly - garishly - coloured red, blue and white; Which definitely give them the look of a child's - or a clowns - toy.