Github is a place where you can freely host ‘GIT Repos’. A GIT repo is the set of source files that power a website or app. Over the years Github has become a thriving community where ad-hoc software teams have formed and developed wildly popular open source software. For example, here's the Github page for Linux and for Bitcoin...
You can literally browse through and download all the source files that make up these projects. You can also see exactly who wrote each line of code and when they committed them. You can see when specific versions were released, you can see any open issues, and any ongoing conversations about how the software is developing. This is the one place on the internet that best shows how modern software works, and how organic and freeform software development can be. You can also, of course, dive in if you see an bug or an opportunity to improve a piece of software. Your changes will be submitted to key project stakeholders, and if accepted will become part of the main codebase.
Github was bought by Microsoft in 2018, but has so far retained the independent community spirit that made it so popular in the first place. There are alternatives to Github however, with the next most popular platform being Gitlab.