It's not exactly part of the webauthn standard. Or at least not a described part, it falls under the 'roaming authenticator' and 'backup credentials' mentioned in the spec, effectively it's a function that generates a crypto key where the key matter is the 24 randomly chosen words. It's basically just _another_ key that a service can setup and store for your, but encrypted with the secret they share with you up front in the form of those words.