FIDO U2F is a second factor authentication specification created by the FIDO Alliance. It outlines detailed steps that must be taken by an authenticator possessed by the user who is requesting access to a digital resource at the time of authentication, as well as the process to verify the authentication is successful on a server side component.
FIDO2 is traditionally leveraged within security keys that contain a private key (typically using ECDSA keys) to sign challenges issued by a FIDO U2F certified server that then verifies the signature using a corresponding public key. With the release of an updated FIDO specification, FIDO2, U2F has been re-labeled as Client To Authenticator Protocol (CTAP1).
"Our employees were falling victim to phishing attacks, so our security team enforced the use of FIDO U2F Devices. Since we started using FIDO U2F tokens as authenticators, we've seen a significant decrease in phishing attacks."