There exists software with source codes that can only be modified by the developers, team, or company that made it. They own the exclusive control or rights over it to edit. The IT industry calls this kind of application as "proprietary" or"closed source" software.
The open-source software or program is the exact opposite of the proprietary software. It's the open source developers or the authors who make its source code available to other people who'd like to see that code. Anyone can copy it, learn from it, change it, or discuss it. The GNU Image Manipulation Program and LibreOffice are examples of open-source software.