Ruby on Rails, often simply called Rails, is a powerful open-source web application framework written in the Ruby programming language. It is designed to make programming web applications easier by making assumptions about what every developer needs to get started. This allows you to write less code while accomplishing more than many other languages and frameworks.

Rails follows two major guiding principles:

• Convention over Configuration (CoC): Rails has opinions about the best way to do many things in a web application, and it defaults to this set of conventions. This means you spend less time configuring files and more time writing your application's unique logic.
• Don't Repeat Yourself (DRY): This principle states that "every piece of knowledge must have a single, unambiguous, authoritative representation within a system." Rails helps you avoid duplicating code, making your application more maintainable and less bug-prone.

It is a full-stack framework, meaning it handles everything from the database (the "back-end") to the web pages served to the user (the "front-end"). It provides a default structure for your database, your web pages, and your web services. Popular websites like GitHub, Shopify, Airbnb, and Basecamp were built using Ruby on Rails, proving its capability to scale for large, high-traffic applications.

For beginners, Rails is an excellent choice because it emphasizes developer happiness and productivity. Its elegant syntax and vast ecosystem of open-source libraries (called "gems") allow for rapid development of secure and feature-rich web applications.