The flash provides a way to pass temporary objects between actions. Anything you place in the flash will be exposed to the very next action and then cleared out. This is a great way of doing notices and alerts, such as a create action that sets flash[:notice] = "Successfully created" before redirecting to a display action that can then expose the flash to its template. Actually, that exposure is automatically done. Example:

class WeblogController < ActionController::Base
  def create
    # save post
    flash[:notice] = "Successfully created post"
    redirect_to :action => "display", :params => { :id => post.id }
  end

  def display
    # doesn't need to assign the flash notice to the template, that's done automatically
  end
end

display.erb
  <% if flash[:notice] %><div><%= flash[:notice] %></div><% end %>

This example just places a string in the flash, but you can put any object in there. And of course, you can put as many as you like at a time too. Just remember: They’ll be gone by the time the next action has been performed.

See docs on the FlashHash class for more details about the flash.