To turn in means to submit or hand over something, usually to a person in authority. When your teacher asks you to turn in your homework, you're giving it to her so she can grade it. When you finish a test, you turn it in at the teacher's desk. Libraries ask you to turn in books by their due date.

The phrase creates a picture of handing something inward, toward someone or into a collection system. You might turn in a permission slip for a field trip, turn in a lost wallet you found, or turn in your uniform at the end of sports season.

The phrase has another meaning too: to go to bed. When you're tired at night, you might say you're ready to turn in, meaning you're heading to your room to sleep.

People also use turn in to mean reporting someone for doing something wrong, like if you saw someone stealing and decided to turn them in to the principal. In this case, you're handing over information to an authority figure who can address the problem fairly.