The meaning of this term is very simple: it is something people can wilfully modify according to their own needs or wishes. Firstly, this term was known in the context of software, which code was publicly exposed and available for modification. Later open source spread widely: there are open source projects, products, participations and many others.
Many organizations and people choose open source software, hence it is considered to be more secured and grants people more control over it. This software can also be more stable as many other people may contribute their own ideas, correct it or improve it.
There is this general idea that open source is for free. That’s is not completely true. The open source product itself is usually free and the creators usually charge other organizations for support or software services as implementation or deployment.
Example
There are many open source softwares or systems available on the market. One of the best known is perhaps Linux, a computer operating system with the penguin logo. Anyone can download it and adjust its code according to one’s own needs.