Open Source software
Open Source software is software of which the source code is freely available to everyone.
Generally Open Source software is developed by groups of people working together.
There are many many projects that use Open source software. A few well known ones are Linux, Android and Mozilla Firefox.
All Muniam firewalls use the open source operating system OpenBSD.
The internet runs on it
Most major websites run on Open Source software. Facebook, Google, Yahoo all depend on open source software.
WordPress the biggest content management system for websites is Open Source software.
Open Source projects are behind behind the Google Chrome and Mozilla Firefox browsers.
Because many people work together on the software and because the source code is available for anyone to look at bugs are typically found and fixed much more quickly than with proprietary software.
For the same reason Open Source software is not easily subject to interference by nation states or hackers.
With Open Source you don't have to trust that a vendor has implemented security properly, you can actually verify it!
Open Source software uses and promotes open standards. This is because the software is not proprietary and is often shared between different projects.
So good interoperability and no vendor lock-in!