Explanation

• root 'topics#index' is a Rails route that says the default address for your site is topics#index. topics#index is the topics list page (the topics controller with the index action).
• Rails routes control how URLs (web addresses) get matched with code on the server. Similar to how addresses match with houses and apartments.
• The file config/routes.rb is like an address directory listing the possible addresses and which code goes with each one

• routes.rb uses some shortcuts so it doesn't always show all the possible URLs. To explore the URLs in more detail we can use the terminal.

  In the shell window type rails routes -c topics. You should get something that looks like this:

  $  rails routes -c topics
        Prefix Verb   URI Pattern                Controller#Action
        topics GET    /topics(.:format)          topics#index
               POST   /topics(.:format)          topics#create
     new_topic GET    /topics/new(.:format)      topics#new
    edit_topic GET    /topics/:id/edit(.:format) topics#edit
         topic GET    /topics/:id(.:format)      topics#show
               PATCH  /topics/:id(.:format)      topics#update
               PUT    /topics/:id(.:format)      topics#update
               DELETE /topics/:id(.:format)      topics#destroy
          root GET    /                          topics#index
  

  This shows all the URLs for the topics_controller. The code that starts with colons are variables so :id means the id number of the record. The code in parenthesis is optional.

  You can also get this information on your site in development. Go to your servers URL plus /rails/info and you'll see something like this:

  

  You'll also see that table in whenever you try to access an invalid route (try your servers URL plus /sandwich)