A product key is a 25-character code that's used to activate Windows. It 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.
The location of your product key depends on how you acquired your copy of Windows:
From an authorized retailer: The product key should be on a label inside the box it came in.
A new PC with Windows: The product key is pre–installed on your PC and your PC will automatically be activated. The product key should be included with the packaging the PC came in, or included on the Certificate of Authenticity (COA) attached to the PC.
A digital copy from a Microsoft website: The product key should be in the confirmation email you received after buying it. If you bought a digital copy of Windows 10 from a Microsoft website, the product key should 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.
Free upgrade to Windows 10 from Windows 7 or Windows 8.1: You'll receive a digital entitlement for Windows instead of a product key.