The term "open source" refers to something people can modify and share because its design is publicly accessible.
The most important aspects of open source are:
- Free to use
- Code open to inspection
The first one means that you can download and use open source software for free. IDEs, web servers, database servers, operating systems, browsers, media players… lots of things.
The second one means that you can look at the code. You can check for bugs, and security holes. You can learn how the code works, improving your expertise.
The last one means that you can modify the code to suit your own needs. Different licenses limit how you can share the result. The most liberal licenses let you build your own product on open source code, and sell the result at a profit.