A domain authorization code (often referred to as an auth code or an EPP code) is a unique set of characters assigned by the registrar at the time of registration to a gTLD such as .com. It acts as a password to prove your ownership of the domain and authorize a domain transfer.
Only individuals and organizations with the proper permissions, such as the registrant contact, can access the authorization code for domain transfers. If a wrong auth code or EPP code were provided when setting up a domain transfer, the transfer would not proceed.