A YouTube activation code is a code that is generated by the YouTube TV app on certain devices, such as an Apple TV or other connected TV, and is used to sign in to a user's Google account. The activation code is part of the first step of the sign-in process.
To receive a YouTube activation code for the app, users need to choose a Google identity that is associated with their YouTube account. This identity can take the form of either a Google+ account or an email address. Once an account has been chosen, the user is prompted to give it access to their TV and YouTube app.