What is open-source software
Wikipedia defines open-source software as follows:
"Open-source software is a type of computer software in which source code is released under a license in which the copyright holder grants users the rights to study, change, and distribute the software to anyone and for any purpose."
The related open-source software development is a collaborative form of software development involving multiple contributors. Together they create and maintain software and source code using open sources. The use of open-source software is widely adopted now.
Microsoft itself has also-embraced open-source software in its software and the development platforms they offer.
The .NET platforms, such as the original .NET Framework and even more so .NET Core, use several components created by the open-source community and not Microsoft itself. In ASP.NET and ASP.NET Core, many of the frontend development libraries are open-source components, such as jQuery, Angular, and React.
Instead of creating new components themselves, the teams at Microsoft are using the open-source components and taking a dependency on them.
The teams also contribute and invest in the open-source components and projects, joining in on the collaborative effort. Besides adopting external open-source software, Microsoft has also made significant parts of its software available as open-source.
.NET is a perfect example of how Microsoft has changed its posture towards open source. It has made the codebase for the .NET Framework and .NET Core available and many other components.
The .NET Foundation aims to advocate the needs and evangelize the benefits of the .NET platform. And promote the use of .NET open source for developers.
For more information, see the .NET Foundation website.