This is a lens for the iPhone 4 and iPhone 4S that clips on.
It makes it possible to do the fisheye and wideangle-type lenses. Basically, it enhances the picture taking abilities of your handset. It’s limited to the iPhone 4 and iPhone 4S. So it can’t be adjusted to fit your iPhone 5. That’s not a total surprise, but you can’t use it with a case, either. That’s a bit of a bummer.
It’s about $70, and whether that’s reasonable is subjective. It’s an interesting accessory if you want to improve your iPhone camera.