Lacan's concept of Id is based on Freud's initial use of the concept. Freud's model of the psyche divided it into three agencies: the Id, the Ego and the Superego. Lacan's use the concept was a bit different than Freud's. Lcan viewed the Id as the origin of the subconscious in speech, the symbolic "it" behind the imaginary I. being symbolic in nature, Lacan identifies the Id with his notion of subject, that is the symbolic order.
see: Lacanian Terminology