Question

Can you explain me what these lines of code do ?

resources :posts, :pages do
      post 'preview', :on => :collection
    end

resources :undo_items do
      post 'undo', :on => :member
end

Answer 1

resources :posts, :pages do

will create standard/default RESTful routes for posts and pages (GET POST /posts, GET PUT DESTROY /posts/1, GET /pages/1/edit and etc).

 post 'preview', :on => :collection

will add additional route for collection. POST /posts/preview. if it said :on => :member than route would be for a member and would look like this POST /posts/1/preview.

That’s a quick guide so you can get your head around. But as Robert K suggested, I also highly recommend going over rails guides. They are very well written and easy to understand.