You're getting a bunch of good, accurate answers, but I want to give it a shot, too. Keep is mind I'm grossly generalizing.
Computer programs are compiled from source code. Programmers keep track of the history of the changes they make, the different versions of the software, with version control systems. Github is a centralized place developers can upload their source code so that other developers can use it.
It's not really intended for end users to use directly. It's a file sharing service, but for source code. That said, github has the capability to compile programs if there developers set it up, and then you get compiled programs you can download from it.
I would suggest not trying to use github directly, unless you're looking to get into programming, or if there's software there you want to try that isn't available from your distribution's official software channels. It's not really an "app store;" it's a Microsoft-owned software service for developers that is sometimes used to host readable documents, because it can be used for collaborative sharing of versioned text files.
There are a great many exceptions to what I said, but that was essentially how github started life, even if it's used for other things and has more features now.