To get Rails saying "Hello", you need to create at minimum a route, a controller with an action, and a view. A route maps a request to a controller action. A controller action performs the necessary work to handle the request, and prepares any data for the view. A view displays data in a desired format.
In terms of implementation: Routes are rules written in a Ruby DSL (Domain-Specific Language). Controllers are Ruby classes, and their public methods are actions. And views are templates, usually written in a mixture of HTML and Ruby.
When an action does not explicitly render a view (or otherwise trigger an HTTP response), Rails will automatically render a view that matches the name of the controller and action.
Source: https://guides.rubyonrails.org/getting_started.html#say-hello-rails