If you choose to reserve your Windows 7 or Windows 8.1 PC to get the free upgrade to Windows 10 before its launch date on July 29, Microsoft will pre-load files for the new OS on your machine ahead of the release.
In a Windows 10 FAQ page, Microsoft states:
When you reserve, you can confirm your device is compatible with Windows 10. Between reservation and when your upgrade is ready, the files you need for the upgrade will be downloaded to your PC to make the final installation go more quickly. Then, when your upgrade is ready after July 29, 2015, you get a notification that lets you get started with your upgrade.
The FAQ page also mentions this about how long the installation should take for Windows 10 after the pre-load is complete:
Plan for about an hour for the installation. Newer devices may only take 20 minutes, and older devices may take more than an hour.
Of course, pre-loading is old news for gamers who can do the same thing when they pre-order digital versions of some Xbox One games before they are released.