The activation key is a verification key that is created for every product key and computer id combination. Normally when you activate your product you need to know only your product key. When the automatic activation doesn't work you need to activate your product manually. For that you simply follow these steps and you copy and paste the created activation key from the web browser to the activation window. You do not need to store your activation key after the activation.
You may see your own activation key from Help > About.
