You can choose to make your book available immediately or set a future release date with pre-order availability.
If you choose to enable pre-orders, customers will be able to pre-order your e-book for up to a year in advance of the publication date. An Amazon sales page will be created in order for customers to pre-order the book. The book will not be available for download until the publication date. On that day, Amazon will automatically send your e-book to all customers who pre-ordered it.
Amazon enforces strict rules around pre-orders. For example, if you choose to cancel a pre-order, perhaps by deciding not to publish your book, you will be prohibited from setting up new pre-orders for one year.
Find out more about pre-orders here: https://kdp.amazon.com/en_US/help/topic/G201499380