Open source is software industry jargon to describe computer code, written to make software programs work, that is available in the public domain. Application developers and in general, anyone from the public can download, use or modify it to suit their purposes.
For example…
The browser Firefox is an open source software. The company behind it, Mozilla has made the code behind the browser public for platforms including the Windows operating system as well as iOS. This enables many software programmers to build upon the functionality offered by the browser or use it as a platform on which their software runs.