Yubikeys are USB keys marketed by Yubico. What makes them interesting is that they are physical authentication devices. Several options are available: U2F, 2FA, PGP, etc.
These keys are used by many businesses and professionals to add a layer of physical security. Only the holder of the key can access the account in question using a U2F (Universal Second Factor) code.
This U2F code is usually sent to a telephone via SMS or a dedicated application, or by email. Here, the “second factor” of authentication is generated by the USB key, which implements a dedicated chip (similar to the Smart Cards on your bank cards). Simply plug in the USB key and press a button to generate a unique code. In this case, the code is generated by the Yubikey. In particular, it is impossible to use MITM (Man In The Middle) attacks like with a mobile phone if you receive the code by SMS or via an application. Similarly, your accounts are not compromised if your e-mail address is hacked.
Many services are now natively compatible: Facebook, Twitter, Youtube, Github, GitLab, BitBucket, Lastpass, Dashlane, Dropbox, Google Drive, Onedrive, most browsers and many more.
The advantage of a physical solution is that it cannot be hacked. The idea that an attacker could remotely access and compromise an account is reduced to nothing. Of course, a key can be lost… but it is possible to have a spare.