A product key is a 25-character code that's used to activate Windows. Activation helps verify that Windows hasn’t been used on more PCs than the Microsoft Software Licence Terms allow.
Your product key should be on the box the Windows DVD came in – or in an email that shows you've bought Windows.
It looks like this:
PRODUCT KEY: XXXXX-XXXXX-XXXXX-XXXXX-XXXXX.
It's a good idea to save a copy of your product key for your records in case you ever need to reinstall Windows.