When and how request sometimes found to be unverified. ruby on rails 3.2

#This is the method that defines the application behavior when a request is found to be unverified. #By default, Rails resets the session when it finds an unverified request.

  def handle_unverified_request
    reset_session
  end

I have seen this explanation at "https://ift.tt/3kGBNgw" now my question is when and how every request sometimes become unverified? how it was hapenning? and when.

thankyou, i have tried to search it but i have seen explanation so deep-technical hence i can understand in an easy way