Let’s start with “closed-source software”, which is a fancy name for most proprietary software that you buy (or supposed to buy). This kind of software comes with a user license that does not allow you to modify the software or to share it with others. Once you purchase the software, you are allowed to use it, but not much else. An example of closed-source software is the popular Microsoft Office suite, which includes Word and Excel. You can use Microsoft Office to edit documents, but you are not allowed to distribute copies of Microsoft Office to friends, and you are not allowed to modify the software. Even if you decided to violate the license and to tweak the software, you would find it to be extremely difficult: the original human-readable code of the program, known as the source code, and written by engineers at Microsoft, is kept secret (or “closed”); the software that you buy is only a translation of that source code into a cryptic sequence of computer instructions that is very difficult for humans to modify.
The open source philosophy has propagated into other domains, such as education, robotics, science, and media. And if you are into beverages, you can even find an open-source cola recipe!