Source code is normally a closely guarded secret of the company that developed it in the first place.
After all, they had to pay software programmers to produce it. Source code is an important part of the 'intellectual property' of a business.
Open Source means that the source code is freely available so anyone can read and alter it as they wish, then compile it to create a new version. If the new version is good enough, then the group looking after that particular 'project' may include it in the next stable release of the source code.
Note that Open Source is not entirely free of restrictions - a 'license' comes with it that defines the way you can use and distribute your adaption. A very popular software licence is the 'GNU General Public License'.
Challenge see if you can find out one extra fact on this topic that we haven't already told you
Click on this link: Open Source