An authcode is typically used during a domain transfer. In some cases you also need it when you want to change the owner of your domain. Whether an authcode is necessary for these actions depends on the respective top-level domain (TLD). On the detail page of a TLD, you can see if (and in what cases) you need an authcode for your domain. You can reach the detail page of each TLD through the overview page of all available top-level domains.
The authcode may have different names, depending on the provider. It can also be found under the synonyms EPP Key, Authorization Key, Authorization Code, Transfer Authorization Code, EPP Code or Transfer Key. However, the key function that this code has - the prevention of unauthorized domain transfers (in some cases unauthorized owner changes as well) - always remains the same.
Each domain has a unique authcode which is only known to the domain owner and the current registrar (provider). In the case of a domain transfer (or owner change) this authcode is specified as the identification means of the current domain owner. This can prevent potential abuse.
The authcode can always be obtained from the current registrar / provider of the domain.