Open source describes a project whose source code is publicly available and licensed such that individuals can use and alter the source code freely. Projects licensed under the MIT, the BSD 3-Clause or 2-Clause, or the GPL Licenses can all be considered open source.
Open source projects are unique in that any user can simply copy-paste the code into a new project, and make customizations to their liking. This is called forking a project,and is common practice in the open source community. When a user forks a project, they do so without affecting the original project, or any of its users. This allows everyone to experiment on their own without negatively impacting others. Once a user has forked a project and added some of their own code, they can let the original project know that they have an upgrade available, allowing the original code to integrate the upgrade as well. This collaborative process allows complete strangers to innovate without worrying about patent infringement.
Bitcoin is entirely open source. The full source code can be viewed on the Bitcoin GitHub page. This allows hundreds of developers from all over the world, some anonymous, to contribute and review code for Bitcoin. Many individuals and corporations fork Bitcoin’s source code to add custom features, but this does not affect the users. Additionally, almost all Bitcoin wallet software and other Bitcoin related software, including the Lightning Network is open sourced.