Preloading means you can download a game before it’s released, so you can play it as soon as it becomes available, and then get to work late the next morning because you had a “stomach bug”. Apparently, while the Epic Games Store does support preloading, they’re “not certain it’s up to the demands of a blockbuster like Borderlands”. That doesn’t inspire much confidence in EGS’s ability to deal with the flood of release-day downloads.
So, how soon you get to play the PC version of Borderlands 3 depends pretty much entirely on your download speeds. It seems the file size should be around 20 GB.
None of this applies to the console versions, so you should be able to preload those if you want to.
Borderlands 3 is set for release on 13 September for PC (as a timed EGS exclusive), Xbox One, PlayStation 4, and Google Stadia.