Without the original source code, even though you can change the binary a little bit to hack it in small ways, it's practically impossible to make any major change to the program, such as adding a new feature. Some people believe that it's much better to build code collaboratively, so they started to make their source code open for other people to change. That's the main concept about open source: software that everyone can use and modify freely. Linux-based operating systems (such as Ubuntu, Fedora, Debian) are open-source, whereas Windows and Mac OS are closed source.
