What is the meaning of "requires build-time bytecode instrumentation"?

lazy (optional - defaults to proxy): By default, single point associations are proxied. lazy="true" specifies
that the property should be fetched lazily when the instance variable is first accessed (requires
build-time bytecode instrumentation).

CGLib has to work its magic. Don't worry about it, it's the default. So long as you've got cglib.jar on your class path, Hibernate will handle it all for you.