The document discusses hardware-backed authentication keys called YubiKeys. It describes the YubiKey's durable design with no batteries or moving parts that allows it to communicate over USB and support multiple authentication protocols like one-time passwords, smart cards, and FIDO U2F. The YubiKey 4 can support multiple configurations and protocols simultaneously. YubiKeys can be used across many applications and services for two-factor authentication, code signing, and encryption. Yubico, the company that created YubiKeys, has millions of users in 150 countries and provides resources and documentation to support integration and use of YubiKeys.