"Software for which the human-readable source code is available for use, study, reuse, modification, enhancement, and redistribution by the users of that software." from the US DoD

Just about damn near every piece of technology you interface with in the modern era is based on or uses open source code.  Most mobile applications and any web site you visit is built on or leverages cloud services; and those cloud services in turn are built on open source software.  It is also powering a significant chunk of the US government's digital infrastructure.

Given the prevalence of OSS, there are some distinct entities you should be aware of in that space:

The Apache Software Foundation : the glue holding the world together and the foundation for the advancement of cloud technology.

Github not all open source, but it is the primary storage and version control source for software development.

Ubuntu there are a bazillion Linux distros, but Ubuntu is the 800 lb gorilla, and for good reason.

VS Code unless you work exclusively with a single language, if you're using a different IDE you're probably doing it wrong.