What does the rule event mean "Drupal is initializing"
In Drupal 7, the rule will run when hook_init() is called. As explained on the Drupal documentation page for hook_init():


  This hook is run at the beginning of the page request. It is typically
  used to set up global parameters that are needed later in the request.
  When this hook is called, the theme and all modules are already loaded
  in memory.
  
  This hook is not run on cached pages.
  
  To add CSS or JS that should be present on all pages, modules should
  not implement this hook, but declare these files in their .info file.