What is Layout_by_resource?

It is used in mostly all rails app but I couldn't find its meaning. I need to understand this code:

class ApplicationController < ActionController::Base
  layout :layout_by_resource

  protected

  def layout_by_resource
    if devise_controller?
      "layout_name_for_devise"
    else
      "application"
    end
  end
end

And this:

Rails.application.config.to_prepare do
  Devise::SessionsController.layout "devise"
  Devise::RegistrationsController.layout proc { |controller| user_signed_in? ? "application" : "devise" }
  Devise::ConfirmationsController.layout "devise"
  Devise::UnlocksController.layout "devise"
  Devise::PasswordsController.layout "devise"
end