An EPP code or the Domain Authorisation code (Auth Code) is like a password for a domain. It provides extra security for a domain name. It is assigned to the domain by the registrar during registration.
For you to transfer a domain from one registrar to another, you need the EPP code Without it, transfer cannot be initiated.
To get the EPP code, you can ask your current registrar to provide it via email. Some registrars allow you to fetch the EPP code form your client area.
An EPP code can expire after some while if it was generated and not put to use for approximately two weeks.