The short version
Most of the software you use daily is built on open source. Linux runs most of the internet's servers. Android is open source. The tools that build websites, train AI models, and run cloud infrastructure are overwhelmingly open source.
The idea is simple: if the code is public, more eyes find more bugs, more people contribute improvements, and no single company controls the tool. It's a bet that openness produces better software, and it's been winning that bet for decades.
How it works
Open source isn't just "code you can see." It's defined by licences, legal documents that specify what you can do with the code:
- MIT: do whatever you want, just include the copyright notice. The most permissive.
- Apache 2.0: like MIT, but also grants patent rights. Common in enterprise software.
- GPL: you can use and modify it, but if you distribute your modified version, you must also make your code open source. This is "copyleft."
The ecosystem runs on contributions:
- Maintainers: the people who manage the project, review code, and decide what gets merged
- Contributors: anyone who submits bug fixes, features, or documentation
- Issues: bug reports and feature requests, usually on GitHub
- Pull requests: proposed changes that maintainers review before merging
Most open-source projects live on GitHub. Some of the largest (Linux, Kubernetes) have thousands of contributors from hundreds of companies.
Why it matters
Open source is how most modern software gets built. Many AI models (Llama, Mistral, Stable Diffusion) are open source. Businesses build on open-source foundations and contribute back. Understanding open source helps you evaluate tools, contribute to projects, and understand the economics of modern software.