The Auth Code (short for Authorization Code) is sometimes referred to as the AuthInfo Code. Other terms for the same subject are EPP code (short for Extensible Provisioning Protocol code), EPP Authorization Code, transfer secret, or transfer key. This is a type of domain password only known by the registered user or owner of a domain name. It consists of 6 to 16 characters and is a combination of letters and numbers.
When a domain is transferred from one registrar or provider to another, the Auth Code serves to verify your authorization. Your new provider will require this verification to host the domain on their server. This is the only way they can be sure that you are the registered owner and avoid running the risk that it is an illegal domain transfer. Requesting the Auth Code is the first and most important step of every domain transfer.
Request the Auth Code or EPP code from the registry
If you registered your domain yourself with the designated registry for your top-level domain, you need to request the Authorization code for your domain directly from the registry. For example, if you have a domain ending in .com (a top-level domain), then that registry would be Verisign, which allows you to manage your domain yourself, using their Domain Tools.
For other country-code domains (e.g. .fr or .co.uk) and newer generic domain endings like .shop, .music or .nyc, you will find specific registries designated for each case. You can consult ICANN’s Registry Listings page to find your registry.