Ruby on Rails is not a minimalist framework, it's a metropolis. One filled with all the major institutions needed to run a large, sprawling application like Basecamp or GitHub or Shopify.
This means there's a lot to learn! But it also means you can learn it piece by piece. You do not need to be an expert in all facets of Ruby on Rails to get started on creating a great application.
The spine of a Rails application is the Model-View-Controller pattern that organizes programming logic into three major layers. The model is where your so-called business logic lives. This is where you calculate whether the next invoice is due or how many todos are left to be completed on a project. The controller is what accepts clicks and taps from the user, routes the commands to the model, and then renders HTML or JSON templates as the view.
In Rails, the model is chiefly handled by Active Record (with some backup from Active Model). That's the framework for talking to the database. The controller is handled by Action Controller and the view by Action View, together presented as Action Pack.
In addition to these core frameworks, RoR also ship with Action Mailer, for sending HTML and plaintext emails. Action Cable for making live updates to pages over WebSockets. Active Support enhances the Ruby language and standard library with more sugar and goodies for use in web applications. And finally, Railties provide all commands, generators, and glue to bind all these frameworks together into the cohesive system that is Ruby on Rails.