Microsoft is coming up with great ideas about Windows 10 and now they are trying to make upgrade to Windows 10 even faster and easier. This can be done by reserving your upgrade for Windows 10. Microsoft explains more about it in the FAQ page by answering as:
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.
So by reserving you are not only getting a free upgrade but also a faster upgrade. But before you go ahead and reserve your upgrade, do check if your PC has the minimum requirement needed for the OS.