Intercept is a TIdServerIntercept property that allows a developer to create TIdIntercept descendants to encrypt and decrypt data, or to compress and decompress messages handled by the server. 

When a value has been assigned to Intercept, it is used in the Listener thread to accept a new connection and allocate the matching Intercept required in the OnBeforeRun event handler of the client context. 

One common use of TIdIntercept is to provide support for the Secure Socket Layer (SSL) protocol, using a TIdServerInterceptOpenSSL instance, in TIdCustomTCPServer and descendants.

TIdServerIntercept 

TIdConnectionIntercept 

TIdContext.OnBeforeRun 

TIdServerInterceptOpenSSL 

TIdConnectionInterceptOpenSSL