In this Lightning Talk presented by Dorian Marie at RailsConf 2021, the main theme revolves around understanding package management within Ruby on Rails applications using the Packwerk tool.

The key points discussed include:

- Introduction to Packwerk: Dorian explains what a package is—a group of classes and files—and how it can help manage services and components within an application.

- Importance of Package Isolation: The talk emphasizes the need to isolate packages. This isolation can make it easier to reason about code, understand its structure, and facilitate easier modifications and refactoring.

- Technical Concepts: Dorian introduces two primary concepts in package management: privacy and dependency. Privacy issues arise when a package tries to access a class within itself, whereas dependency issues occur when a class from one package calls another class from an external package.

- Setting Up Packwerk: The process of setting up Packwerk is straightforward. Dorian outlines the necessary steps including installing the gem, setting up the package.yml file, and organizing application components into distinct packages.

- Public Classes and Access Control: The distinction between public classes, which can be used throughout the application, and private classes is crucial for maintaining clean architecture. Dorian discusses how public classes should be organized and notes the importance of enforcing dependencies in the package management system.

- Running Packwerk Checks: The talk describes how to perform checks on the package's structure through Packwerk. This ensures that there are no incorrect dependencies or privacy issues in the code when referencing classes across different packages.

Throughout the presentation, technical details are provided alongside real-world application scenarios, helping to illustrate the practical benefits of using Packwerk in a Rails app. The closing remarks encourage viewers to delve deeper into Packwerk and explore its documentation on GitHub and the speaker's website. The overall takeaway emphasizes the organizational benefits and improved code maintainability that accompany properly managed package systems in Rails applications.