To hand in something means to give it to someone in authority, usually to fulfill a requirement or complete a task. When you hand in your homework, you're giving it to your teacher to be checked or graded. When a library book is due, you hand it in at the front desk.

The phrase suggests a formal transfer from you to someone who's expecting it. You don't just hand in things to your friends; you hand them in to teachers, coaches, bosses, or officials. A student hands in their permission slip for the field trip. A detective hands in their badge when they retire. An employee hands in their resignation letter when they quit their job.

Notice that handing something in often marks completion: once you've handed in your science project, that part of your work is done. It's now in someone else's hands to review or process. The phrase can also mean turning something over that you're required to give up, like when students hand in their phones at the start of a test.

The opposite is usually to get back whatever you handed in, like when your teacher returns your graded essay or the librarian gives you a receipt for your returned book.