FIDO2 is an open authentication standard, hosted by the FIDO Alliance, that consists of the W3C Web Authentication specification (WebAuthn API), and the Client to Authentication Protocol (CTAP). CTAP is an application layer protocol used for communication between a client (browser) or a platform (operating system) with an external authenticator such as the YubiKey 5 Series, and the Security Key Series by Yubico. Yubico is a core contributor to the FIDO2 open authentication protocol.
FIDO2 is an extension of FIDO U2F, and offers the same level of high-security based on public key cryptography. FIDO2 offers expanded authentication options including strong single factor (passwordless), strong two factor, and multi-factor authentication. With these new capabilities, the YubiKey can entirely replace weak static username/password credentials with strong hardware-backed public/private-key credentials. These credentials cannot be reused, replayed, or shared across services, and are not subject to phishing and MiTM attacks or server breaches.