As online user accounts become more powerful and prevalent, it is important to keep your accounts secure. Many online platforms now offer two-step verification, making it harder for attackers to gain unauthorised account access. One form of two-factor authentication is a Universal 2nd Factor (U2F) security key, which is a physical key used to unlock your online accounts.
See https://mybroadband.co.za/news/security/210374-how-to-lock-your-facebook-account-with-a-physical-key.html 
How to lock your Facebook account with a physical key
As online user accounts become more powerful, it is important to keep your accounts secure.