In last few years domain registry companies have seen customers taking away their .COM / .NET domains freely to other registrar. In the process some frauds also happened. To put a full stop to all this ICANN has made it mandatory for all domain transfers to be supported by a mandatory AUTH-INFO Code (or Transfer Authorization Secret).
To transfer your domains OUT of a Registrar (referred as Loosing Registrar), you have to get your own AUTH-INFO code from the Registrar Company. Some companies provide it within their control panel while some may send it to your email. Then you must provide this AUTH-INFO / Transfer Secret to the new Registrar Company (referred as Winning Registrar), in the way they decide to manage that, to start the transfer. Once started, both winning and loosing registrars may contact you to approve or deny the transfer. If not denied within 5 -7 days, the domain will be transferred to the new registrar.