With vidIQ's 2-factor authentication, if you add your mobile phone number to your vidIQ account and use vidIQ features to make changes to your channel, a verification code will be sent to your phone number. You'll need to enter this code in order to save and apply the changes you've made.
If you don't add your phone number, the 2FA code will be sent to the email address associated with your vidIQ account. This extra security step is only required for certain tools and features that allow changes to be made to the videos that have already been uploaded to your YouTube channel.
This is done to ensure that only the channel owner (you) can make changes to your channel, and not anyone else. I hope this explanation helps you understand how 2FA works with vidIQ.