A security key is an authentication device that strengthens account security when used in addition to a password when signing in. Using a security key is better than receiving codes via phone call or text message because these codes can be phished or intercepted. When you use a security key to sign in, the key will check to make sure you are on the official login.gov website.
A security key is usually a piece of physical hardware, like a USB, that you can carry on your keychain. You can also use supporting software, such as a web browser extension or other services. When choosing a security key, look for compatibility with the FIDO standard.