The only entity definition, added via the module, is the YubikeyUserEntityDefinition. It adds the yubico-related attributes to the UserEntityDefinition. The inheritance goes further to the PrincipalEntityDefinition and the AbstractEntityDefinition:
The defined here property stores the yubikeyPublicId.
The implementation checks the generated yubikey for the user. This guide shows how to generate a publicId for the customer and store it in your database.
For more details, please, refer to the Yubico documentation.