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] = "Post 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 PostsController < ActionController::Base
  def create
    # save post
    flash[:notice] = "Post successfully created"
    redirect_to posts_path(@post)
  end

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

show.html.erb
  <% if flash[:notice] %>
    <div class="notice"><%= flash[:notice] %></div>
  <% end %>

Since the notice and alert keys are a common idiom, convenience accessors are available:

flash.alert = "You must be logged in"
flash.notice = "Post successfully created"

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.

Namespace
Methods
C
N
Constants
KEY = 'action_dispatch.request.flash_hash'.freeze
 
Class Public methods
new(app)

Source: show | on GitHub 

# File actionpack/lib/action_dispatch/middleware/flash.rb, line 233
def initialize(app)
  @app = app
end
Instance Public methods
call(env)

Source: show | on GitHub 

# File actionpack/lib/action_dispatch/middleware/flash.rb, line 237
def call(env)
  @app.call(env)
ensure
  session    = env['rack.session'] || {}
  flash_hash = env[KEY]

  if flash_hash
    if !flash_hash.empty? || session.key?('flash')
      session["flash"] = flash_hash
      new_hash = flash_hash.dup
    else
      new_hash = flash_hash
    end

    env[KEY] = new_hash
  end

  if (!session.respond_to?(:loaded?) || session.loaded?) && # (reset_session uses {}, which doesn't implement #loaded?)
     session.key?('flash') && session['flash'].empty?
    session.delete('flash')
  end
end