This document discusses building a REST API for mobile apps using Ruby on Rails. It demonstrates building out features for user signup, login, managing lists, and list items. Cucumber and RSpec are used for testing. The API is developed within 20 minutes, demonstrating features for user authentication and managing to-do lists and list items. An admin panel is also generated using ActiveAdmin to manage users, lists, and items from the backend. The document encourages building a JavaScript frontend app using Backbone.js to consume the API and work with the data.