The Domain AuthCode is also known as the Domain Authorization Code. It is a 6 to 16 character code assigned to your domain name. This code is assigned by your registrar and is simply a password for your domain name. The password is used as a security and authorization measure in events such as a domain transfer.
When you initiate a transfer registrar request, the registry will send you a transfer authorization request. You need to reply to the authorization request with your Domain AuthCode. Only transfer authorizations received with the correct Domain AuthCode will be approved.
If your domain is registered with Doteasy, you can obtain your Domain AuthCode via your Member Zone.