To turn something in means that you submitted it. For instance, a student turns in their homework to their teacher (Entregé mi tarea) To turn someone in means that you informed the police or another authority about that person's behavior (Delaté a mi vecino a la policia )

To turn something in: to submit something for review or approval, like a school project or a job application. To turn someone in: to notify authorities that someone has committed a crime or committed some other infraction.