The ability to allow pre-orders of your books is a new function on the Kindle Direct Publishing platform. It sounds like a pretty handy thing to offer, but before your rush into it, maybe you should read about Derek Murphy’s experience with getting banned for a year due to an error in the system: How to use Kindle pre-orders to pre-sell your self-published book
Advice from Derek:
Make sure you upload a final version of your book at least 10 days (2 weeks to be safe) before the publishing date, since the consequences appear to be dire.
Much Happiness and Success!