A product key is a 25-character code that's used to activate Windows and looks like this:
PRODUCT KEY: XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
Activation helps verify that your copy of Windows hasn't been used on more PCs than the Microsoft Software License Terms allow.
How to find your product key depends on how you bought your copy of Windows 10:
If you bought Windows 10 from an authorized retailer, the product key will be on a label inside the box it came in.
If you bought a digital copy of Windows 10 from a Microsoft website, the product key will be in the confirmation email you received after buying it or will be distributed digitally as an entitlement, in which case you won't receive a product key. For example, if you upgraded to Windows 10 for free, you'll receive an entitlement instead of a product key.
If you bought a new PC running Windows 10, the product key will be pre–installed on your PC and your PC will automatically be activated, included with the packaging the PC came in, or included on the Certificate of Authenticity (COA) attached to the PC.
For more info about genuine Windows, go to the How to Tell webpage.
If you're having problems activating Windows 10, see Why can't I activate Windows 10?
If you're unable to find your product key or need additional help activating Windows 10, go to the product activation article.