A Pre-order is a release available for purchase ahead of its official release date.
However, it will not be available for you to download until the 'Pre-order for' date.
A release on Pre-order is denoted by the pink 'PRE-ORDER' tab listed across the release artwork. The purchase button with the releases price will also be pink.
You will receive an email notification when the track is ready to download, but either way your Pre-order will also always be available on the release date in your downloads page (must by logged in).