Closed source (or proprietary software) means computer programs whose source code is not published. The source code is not shared with the public for anyone to look at or change. Closed source is the opposite of open source.
Most companies who sell their software for money make it closed source so people cannot easily change it or copy it for free.
Even some who give their software for free do not show the source code, because they think it looks bad, or that somebody will change the authors' names to their own (a kind of plagiarism) without making the program better. Some programs called “freeware” are given away for free, but they are not the same as free software that anyone can do whatever they want with.