Github- I don't get it!

I feel super dumb asking this. But what actually is and how does github (or similar sites) work? Are they all just source files one needs to manually compile? I am always confused when I look at a github page. I know some have directions but they still go way over my head sometimes. Im not a total noob but some of this stuff seems like you need to be in programming and have an IDE just to run a program.