Here's a blurb about it from Amazon's site:
Want more information, follow this link to the KDP Select section of the Kindle Direct Publishing site.
"Introducing KDP Select - a new option to make money and promote your book. When you make your book exclusive to Kindle for at least 90 days, it will be part of the Kindle Owners' Lending Library for the same period and you will earn your share of a monthly fund when readers borrow your books from the library. You will also be able to promote your book as free for up to 5 days during these 90 days."