The official definition of Open Source consists of 10 separate, “not short” sentences that are stating “what must open sources be like”. It’s very helpful to understand the context of what Open Source is.
However the word itself was originated to designate a specific approach to creating computer programs, now “open source” covers a range of contexts including open exchange, collaborative participation, rapid prototyping, transparency, meritocracy and community-oriented development. Basically the term is refered to something that others can modify and share because of its design is publicly accessible.
The roots of open source go back to computer science practices in the 1960s in academia and early computer user groups. Computer programmers frequently and informally shared code that they had written (“hacked”), quickly recycling and freely modifying code that solved common technical problems. [britannica]
People prefer open source software to proprietary software for a number of reasons, including:
Many people prefer open source software because they have more control over that kind of software. You can examine the code to make sure it's not doing anything you don't want it to do, and you can change parts of it you don't like. Users who aren't programmers also benefit from open source software, because they can use this software for any purpose they wish—not merely the way someone else thinks they should.
Other people like open source software because it helps them become better programmers. Because open source code is publicly accessible, anyone can easily study it as they learn to make better software. Also when people discover mistakes in programs' source code, they can share those mistakes with others to help them avoid making those same mistakes themselves.
Some people prefer open source software because they consider it more secure and stable than proprietary software. Because anyone can view and modify open source software, someone might spot and correct errors or omissions that a program's original authors might have missed. And because so many programmers can work on a piece of open source software without asking for permission from original authors, they can fix, update, and upgrade open source software more quickly than they can proprietary software.
Many users prefer open source software to proprietary software for important, long-term projects. Because programmers publicly distribute the source code for open source software, users relying on that software for critical tasks can be sure their tools won't disappear or fall into disrepair if their original creators stop working on them. Additionally, open source software tends to both incorporate and operate according to open standards.
Open source software programmers can charge money for the open source software they create or to which they contribute. But in some cases, because an open source license might require them to release their source code when they sell software to others, some programmers find that charging users money for software services and support (rather than for the software itself) is more lucrative. This way, their software remains free of charge, and they make money helping others install, use, and troubleshoot it.