Security keys are part of an extra Security feature called Login Approvals. If you own a Universal 2nd Factor (U2F) compatible security key and turn on login approvals, you can use it when logging into your Facebook account from a new computer or mobile device.
Using your security key is much like using a key to unlock a door. After entering your password, you can tap your physical security key instead of entering a special security code. Keep in mind security keys only work with certain web browsers and mobile devices, so you'll also need to have another login approval method, such as a mobile phone or Code Generator
Using Your Security Key
If you have login approvals turned on and added a security key, the next time you log into Facebook from Chrome or Opera on an unrecognized device you'll be asked to tap your security key.
If you don't have your security key or it isn't working, you can always click “Use a different method” to log in using one of your other login approval methods, such as a mobile phone or Code Generator.
Step 1: Login to Your Facebook account
Step 2: Go to Settings which is right below Logout
Step 3: Now go to Security and Click on Login Approvals
Step 4: You need to select Security Keys – Click on “Add Key”
Step 5: If you have a USB security key, you can use it to protect your Facebook account, Click on “Add Key”
Step 6: Insert your security key into a USB port. If the key has a blinking light, press the button. That’s it.
Step 7: You need to Re-enter your password to save changes, that’s it. You entered a new security method to login to you facebook account