Each theme must contains the .info file as Drupal parses this file to gather all the necessary information about a theme. As the extension suggestss , it provides all the information to Drupal about theme like theme name, Drupal version compatibility, regions declared. css stylesheets used, javascripts files included. .info file also provide the information about the theme engine being used by the theme.Theme engine allow theme developers to communicate to the drupal using a simpler and more convinient interface commonly through template files