KDP Select is a special program offered by Amazon to people who have published e-books for the Amazon Kindle. In order to participate you must agree not to publish your book in any other format besides the one sold by Amazon for the Kindle or kindle apps for a period of at least ninety days. In return the program offers authors two things. Amazon allows premium members to borrow one e-book a month, and they set aside a pool of money to be divided among the authors whose books are borrowed. If you are in KDP select people can borrow your book, and you will get a share of the money, depending on how many do so. They also allow you to give your book away for free for up to five days out of every ninety. While this may seem a dubious privilege, it can be effective as a marketing tactic, and used properly can increase paid sales. On the other hand, if Amazon discovers that you are giving away your book in another format for free, they will automatically reduce their price to match, so if someone chooses they can distribute their book for free on Amazon without accepting the restrictions of KDP Select.
It is relatively easy to use KDP Select to get a free e-book onto one of Amazon's best seller lists. It is relatively easy to get people to take a chance on downloading a free e-book, and indeed many browse the Amazon MOA forum for just that purpose. They maintain separate best seller lists for free and paid e-books, so it is much more difficult to stay on the best seller list after ending a free promotion.