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 License Terms allow.
Your product key should be on the box that the Windows DVD came in—or in an email that shows that you 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.