According the website, Github is a development platform. A lot of open source projects are there, and developers go and work on the different projects. It's been a way for a lot of these projects to move forward without major expense (though I imagine there are people getting paid, like STEEM developers), and to allow others to look at code that works and potentially incorporate them, if permitted, into their own projects.
As mentioned above, STEEM is among the projects with different entries on Github. Pull requests to check code can happen there, discussions about what to include and what not to include, as well as analysis of how things are going can happen there. It puts all of this more or less in one file or place for anyone to go look and see what's happening.
Towards the latter part of October, 2018, Microsoft actually bought Github. Before that, it was a community repository only. According to a post by Microsoft on their own blog site, the intent is:
"Github will retain it's GitHub will retain its developer-first ethos, operate independently, and remain an open platform. Together, the two companies will work together to empower developers to achieve more at every stage of the development lifecycle, accelerate enterprise use of GitHub, and bring Microsoft’s developer tools and services to new audiences."
As far as I know, STEEM continues to be worked on through Github, along with many other open source projects. Whether that will change probably depends on what Microsoft does. If they keep hands off, while working with the huge community of developers, projects will probably continue, even though some will inevitably leave for some other public site.