A product key is a unique code that is used to activate and validate software. It is typically a combination of letters and numbers, and it is often found on the packaging or documentation that comes with the software. In the case of Steam, a product key is required to activate games that have been purchased through the platform.
If you have lost or misplaced your Steam product key, there are a few ways to find it. One way is to check your email inbox for the confirmation email that was sent to you when you purchased the game. The product key will be included in this email.