What is Open Source?
Open source software is where the code of the software is openly available for public access, as opposed to propriety software where the code is often encrypted and not able to be edited.
There is usually no license fees to use the software although some developers ask for a “membership fee” to access the support, updates and downloads to assist with the ongoing cost of further development. This is usually minimal in comparison to propriety software.
There are several advantages to developing open source software. For one thing, it allows developers to tap into the collective knowledge of a global community of developers. When everyone has access to a project's codebase, more eyes are on the lookout for bugs and security vulnerabilities. That means they're more likely to be found and fixed quickly.
Open source also encourages collaboration and facilitates innovation. When developers are able to build on each other's work, they can create something that's greater than the sum of its parts. And because anyone can use and improve open source software, it levels the playing field between large organizations with deep pockets and small startups or individual developers.
Of course, there are also some disadvantages to open source development. For example, because anyone can access and use the code, it can be difficult to protect your intellectual property if you develop open source software. But on balance, the advantages far outweigh the disadvantages.
Read more about open-source on the following link.
https://en.wikipedia.org/wiki/Open-source_software
Both WordPress and Joomla are open-source Projects.