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.
The major advantage of open-source software is that developers can develop addons and extensions, often freely available, to support the open-source project. Read more about open-source on the following link.
Both WordPress and Joomla are open-source Projects.