Proprietary software

Proprietary software (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: 
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 and change the software, or to use the source code to make similar software. It helps prevent a user from pretending to be the author of someone else's work (a way of plagiarism).