When you buy a shiny new PC sporting a copy of Windows, you’ll find it comes with a key for
product activation. You’ll also probably need keys for everything else you buy further down the line. Reasons for registering could include customer support or simply keeping a program functional after a 30 day trial period.
Microsoft products have a very specific way of doing this, and it involves
Product Identification Numbers (PID for short. Don’t confuse these with
Process Identifiers). As per their website on the subject:
A PID is created after a product is successfully installed. PIDs are used by Microsoft Customer Service to help identify the product when customers engage Microsoft for support. A Product key is a unique combination of numbers and letters that is used during Microsoft software installation to "unlock" or open the product. If you do not enter the product key when you are prompted during the installation, the product may not open until you enter the product key.