Proprietary (or closed source) software does not let people see or change source code of the computer software. It is the opposite of Free software. Proprietary software is usually created by businesses who want to sell their software, but some programs that are free to use are still proprietary because the user is not allowed to change them. With proprietary software, only the people that make the software can see and change the code.
Even if the people who make the program give the source code to other people, the program will be proprietary if they do not allow to do important things like these:
- change the code,
- give the code to other people,
- give the changed code to other people,
- use the code on a different computer,
- change it to work on a different operating system and give that to other people.
Most companies who sell their software for money (and even some who do not charge money for it) make it proprietary source. This makes it harder for people to copy or change the software, or to take the source code and make similar software.