Open source means the source code which can be freely accessible for modification and redistribution. Open source is mostly referred to a software. Most of the open source software is also licensed with an open source license such as MIT, GPL, Apache, BSD etc. An open source software may also be a free software, depending the license. Open source software is popular because it is free to use, change or distribute. There are also different open source operating systems such as Linux, Unix, BSD, and Solaris.
Plenty of commercial software may also be open sourced. For example Red Hat Enterprise and Oracle Linux is based on an open source operating system, providing commercial subscription systems for the users.
The Internet is now full of open source CMS software such as WordPress, Drupal, and Joomla. Also it is very easy to find open source e-commerce CMS like WooCommerce, OpenCart, or Magento. The database systems such as MySQL, PostgreSQL, and MariaDB are also open source software. The open-source model is a decentralized software development model which is based on open collaboration. Developers use project management systems and source code providers for an efficient collaboration. GitHub, GitLab and BitBucket are most of the popular DevOps platforms.