As dddddaviddddd said, it's a price tracker for Amazon. To go into a little more detail: you can enter a product name or Amazon URL and it'll show you first and third party pricing history. You can also set a price threshold on a particular product and it'll either tweet or email you when the product price drops below that point. You can even do it without signing up or logging in.

I've set notifications on a couple items that I regularly order and when they drop below a certain price I order enough in advance.