KDP Select is an optional enrollment for e-books.
If you choose to enroll your book in KDP Select, it will automatically be included in Kindle Unlimited. This will affect the way you are paid. Under KU, payment is based on the number of pages read, rather than the number of downloads.
Participation in KU may increase the visibility of your book, as Amazon is more likely to recommend and promote titles available through the program.
However, your e-book (not your paperback or hardback) must be an Amazon exclusive for a minimum of 90 days, or as long as the book is enrolled in KDP Select / KU. While your book is part of KDP Select / KU, you cannot distribute your book on other platforms, such as Apple, Kobo or Google Books.
For more information about KDP Select, visit: https://kdp.amazon.com/en_US/help/topic/G200798990