EPP (Extensible Provisioning Protocol) is the protocol registrars and registries speak. The "EPP code" most people mean is the Authorization Code — a per-domain secret string that proves you own the domain when you transfer it away.
The flow: you want to move yourdomain.com from Registrar A to Registrar B. You request the auth code from Registrar A (usually emailed to the registrant address on file). You paste the code into Registrar B's transfer form. The losing registrar emails you a confirmation; you click "approve". The transfer completes in 5–7 days per ICANN policy.
The auth code is per-domain, single-use, and expires after a window (typically 7–30 days depending on registry). If you don't complete the transfer in time, you re-request a fresh code.
