The form processor is used to handle the submission of a form on an external website in CiviCRM. You can define your inputs (fields) and then define the actions. An action could be create a contact, add contact to group etc...
See https://civicrm.org/blog/jaapjansma/usings-civicrm-form-processor-extens... for an how to use the form processor with an external Wordpress Contact Form 7 or with an external Drupal 7 and Webform website.
Below two gifs of the form processor in action, one with submittng the form from an external Wordpress site and one of the configuration.