Okay kiddo, imagine you have a toy train set with tracks, stations and lots of train cars. Now imagine you can tell the train cars what to do using a special remote control. That's kind of like what Blender Game Engine is!
But instead of trains, we have 3D models of all sorts of things -- from toy cars to trees to buildings. And instead of a remote control, we use programming to tell these models what to do. We can make them move around, rotate, play sounds and even interact with each other.
Using Blender Game Engine, we can build our own little worlds with all sorts of cool stuff. We can also make games where the player controls a character moving through our world, trying to achieve a goal. It's kind of like playing with your train set, but with more power and flexibility to create whatever you want!