How does a game engine/graphics engine work (software)?

I have always wondered how game engines work. It’s not an engine, like a motor or anthying, but simply code if I have understood it correctly. But…well…could someone explain where the differences between different engines (such as CryEngine) and what makes it..well…work? How it works…? I know it’s a complex question, but try to explain as if I am 4 years old :)