Since 2017, Facebook users have been able to use a physical security key to protect their accounts. Users can now setup two-factor authentication on iOS and Android using their security keys.
Two factor authentication relies on something you know, such as your password, and something you have, such as an authenticator app. Hardware security keys can also act as the item that only you have. Hardware security keys are most recommended for users who have a high-risk of hacking attempts. This can be people such as public figures or journalists.
If you want to use a hardware security key you will need to buy one from a compatible manufacturer. You can then go to the Security and Login section of your account settings to get setup.