I'm a mobile game developer so I've done a lot of work with game engines. I'll try to explain what a game engine is in the context of Graphics. You've probably heard of OpenGL and DirectX. These are programming libraries which allow developers direct access to the graphics card. Using these libraries it's possible to load sets of vertices to the graphics card, connect these vertices with lines then polygons and finally assign textures to the polygons. This is how 3D models are created. Maya uses the same approach for it's view ports. Setting up graphics like this is very manual, long winded and complex. To load a 3D shape into the graphics card would require, maybe 300 lines of code. A game engine is a library which makes tasks like this easier. With a game engine you would only need to write 10 lines of code. It takes care of all the detailed low-level programming and provides a nice interface for the game developer. This is just an example for graphics but a game engine would provide support like this for: Graphics, Sound, UI, Networking, Physics, AI and particle effects. If you're interested I've written a more detailed article here: What is a game engine?