A pre-order is a purchase of new content before it has been released! For D&D Beyond, this means that you can use and access your content as soon as it is ‘unlocked’ on the site! For physical local game stores this means they reserve and/or order a copy for you, and they are ready to hand you your new book on Release Day!
For our D&D Beyond Marketplace and D&D Store, a pre-order will generally provide digital content on the relevant Release or Early Access dates, and the physical copy will begin the shipping process closer to the Release Date!