Your publisher ID is the unique identifier for your AdMob account. To protect the security of your account and make it easier for us to find account-specific details, you may be asked to provide this ID when you communicate with Google.
You can find the publisher ID by clicking Settings in the sidebar. It's located in the Account information section.
You'll also find your AdSense Customer ID and Google Ads Customer ID here. These IDs are specific to the AdSense and Google Ads accounts linked to your AdMob account.