Safe deposit boxes are another service that banks offer. Inside the bank vault, there are lots and lots of boxes built into the wall. Each one has one or two locks on it. The bank rents the boxes to anyone who would like to safely store important things like expensive jewelry, money, birth certificates or wills. People like to protect these things in case there is a fire or robbery at their house.
Let's imagine that your grandfather gave you a valuable coin collection. You might want to protect that collection by renting a safe deposit box. You would go to the bank and ask an account manager to rent you a box. That person will ask you what size box you need and then tell you how much it will cost for you to rent it for a year.
Bigger safe deposit boxes cost more than smaller ones. You will be given two keys that are the same. If you ever decide to stop renting the box, the bank wants these keys back. Also, the keys need to be in a safe place so that you know where they are but they wouldn't be where someone could steal them.
The account manager will ask you to sign a card to show that you are going to open your rental box. Then you will be taken into the vault. Your key and then a bank key are needed to open the lock. Neither key can open it by itself. The account manager will pull out a box with a lid on it and take you to a private room where you can put things in or take things out of it. Bank people have no idea what you have in the box and don't want (or need) to know.