An authorization code (so-called EPP Code or Auth Code) provides an extra level of security for domain registration. The domain authorization code is unique to each domain and is assigned by the domain registrar at the time of registration. When a domain name is transferred from one registrar to another, the EPP code will need to be shared.
It is important that the administrative details for a domain are kept up to date as it is likely that a registrar will send the EPP code via email to the listed administrative contact. Although, some registrars will also contain the EPP code on their control panel.
When you request an EPP code from your registrar, they are required to send it within 3-5 days (regulated and allowed by ICANN). However, most will send code instantly. Unfortunately, some of the more unique domains (specific TLDs) may not have EPP codes, and the process of transferring them from one registrar to another is a LOT more complicated and will require that you fill in forms to verify your identity.
In that case, it is the best to transfer a domain within the same registrar through a domain push.