Open source refers to software whose source code is made available to the public. This means anyone can view, modify, and distribute the code, promoting collaboration and innovation. Popular examples of open source software include Linux, Apache, and Mozilla Firefox. The open source model encourages community involvement, allowing developers from around the world to contribute to projects. This collaborative approach can lead to more robust and secure software, as many eyes can identify and fix issues. Additionally, open source software is often free to use, making it accessible to a wider audience.