Many Kindle authors want to give away some of their material for free on Amazon as a promotional strategy. The problem is that Amazon does not allow books to be given away for free. Digital books between 3 and 10 megabytes must be priced at a minimum of $0.99. And in case you are wondering, yes, selling your book at $0.99 is vastly different from giving it away for free. But there is a trick you can use.
Amazon also has the policy of price matching. That is, if a competitor like Barnes and Noble offers your book for a lower price then Amazon will match that price, at least for a while. So what many authors do when they want to give away their Kindle books is that they publish them for free on sites like Smashwords. Smashwords then distributes to Barnes and Noble and other book outlets. Amazon notices that their competitors have listed your book for free and lowers the price to zero for about a week or so at a time.
This has dramatic effects. Some authors report that their free books were downloaded at a rate of hundreds of downloads per hour with some reaching into the tens of thousands during the "free" period. Of course, these authors were not making money, but this is a very effective way to advertise yourself drawing attention to your other books and to your website.