Open-source software is software whose source code is published under a license that allows anyone to use, study, modify and redistribute it.
Open does not mean free of rules: the license sets the conditions. There are two main families:
- permissive licenses (MIT, Apache, BSD): the code can be reused almost freely, including in a closed commercial product
- so-called copyleft licenses (GPL): any modified version that is redistributed must stay under the same license
Open source does not mean free of charge or volunteer-run either. Many projects are backed by companies that earn a living from a model built around the code (hosting, support, advanced features), or by foundations.
Open source runs a large part of computing: Linux, Kubernetes, PostgreSQL, Python, React. For a company, it is a way to avoid vendor lock-in, but licenses and dependency security need monitoring.
Why it matters when hiring
Visible open-source contributions are one of the best signals for evaluating a developer: you can read their code, their review exchanges and how they work with strangers. Their absence proves nothing, as many excellent profiles code under confidentiality agreements. For a maintainer or Developer Advocate role, however, real and sustained contributions are expected.