A safety deposit box is a small metal container into which one places valuables or documents that are too important to lose to fire or theft. These containers are located in a bank, many times in a vault, so it has commercial protection via personnel and alarms. Most bank's deposit boxes have two keys. One is held by the bank and the other is held by the box owner. To gain access to your box, bring the key and some photo ID. Once you present yourself to the bank personnel they will either take your key and theirs to open the outer safety deposit box holder, or you will follow them in and both of you turn the keys at the same time, launching the nuclear missiles and causing the apocalypse.
Once you have the inner safety deposit box, you will be left alone or brought to a small side booth for privacy as you add or remove what you desire. Some folks keep precious metals, jewelry, passports, stock certificates, and other commodities. When you are done, the box is returned to the locked holder in the vault and you will leave with your key.
Many times you get two keys. Don't lose them or they will have to have a locksmith drill out the lock and replace it, all on your dime.
Some banks offer a small safety deposit box to their clients as a perk. Other than those, the boxes are rented out. If the rent is unpaid and they are unable to get in contact with you they will drill out the lock and send everything to the state. The bank is not allowed to keep the contents.