Extensible Provisioning Protocol (EPP) is a set of common protocols used for domain registration in DNS, and it uses a code to verify and validate the authenticity of a domain name registration, to prevent several types of attacks on registrars that facilitate hijacking and impersonation attacks.
"Transfer Secret" is a generic name that covers what the code is and what its used for, independent of any particular implementation (such as EPP). Before there was EPP, each registrar used its own set of interfaces for domain registration, and they may implement some form of "transfer secret", for their non-EPP process, so "Transfer secret" just refers to the idea that some verification code is needed before allowing some actions like registrations and zone transfers, including those used by EPP.