The basic premise of open source software is that the software is made available under an open permissive license, as defined by the Open Source Initiative’s Open Source Definition.
Open source software doesn’t however necessarily mean software that is available for free, or without cost. That said, there is a lot of open source software is in fact freely available, where developers and maintainers have contributed time and effort and have not been compensated. Open source also isn’t just about complete software suites, but also about components and libraries that often are widely used and deployed within commercial enterprise applications.