Auth Code is the abbreviation for authentication or authorization code. For some top-level domains an Auth Code is needed in to order to start the registrar transfer (change of provider).
The Auth Code is the key for your domain name transfer.
The Auth Code is needed with most of the generic and some of the CC-TLDs. (.com, .net, .org, .info, .biz, .cc, .ag, .in, .cn)