Org-mode is not just 'something', it's a powerful Emacs extension that transforms plain text documents into dynamic, interactive, and organized systems for notes, to-do lists, and project management, while seamlessly integrating code execution across multiple programming languages with data flow between embedded computational blocks.

Practical example? You can be annotating a PDF, while having executable code blocks in different languages, e.g., you can run http or sql query, then pass the results into a python, javascript or shell scripting block, and then output the final result into a diagram. So, you'd be reading the book, taking notes and mess around with practical code snippets at the same time.

I have tried so many different note-taking apps, philosophies and techniques over the years. Nothing even comes close to what I can do today in Org-mode. It totally shatters everything else and kicks the shit out of the ballpark.