Linux is famous for being 'open source' in that the source code for Linux plus all the applications is normally available for those who want to view or modify it.
However, not all Linux programs are Open Source. Some are 'closed source' in that the developers who create it would like to keep how they created it a 'secret'. The argument here is that the company or developers have invested time and money in creating a program, charge for their work so why should they go releasing the code to any Tom Dick and Harry.
