The authinfo (also known as EPP code, domain secret, transfer secret or authcode) is a unique, secret code which is used for domain transfer.

Due to ICANN rules its length has to be from 8 to 32 characters, and it should contain at least 1 letter, 1 number and 1 special symbol (such as !, (, %, ^ etc.)

• not every TLD need an authinfo to transfer; some TLDs (i.e., .co.uk ) have different authorization processes for the transfer, and don't need an authinfo;
  
• KEEP IT SECRET! 
  No third party, under any circumstances, should know your authinfo. Transmitting the authinfo code to third parties might result in the domain hijacking.
• It may happen that a domain has an authinfo shorter than 8 characters.
  It happens when a domain is many years old, and the authifo was generated before ICANN stated the minimum lenght of 8 characters.
  This can interfere with domain transfer, and bring the transfer to a failure: so, please ask your losing Registrar to re-generate the authinfo.