What exactly is simple diffusion?

pretend you have a rectangular box. At one side of the box, you will have lots of marbles (or something), and on the otherside, you dont have much or any at all. The whole point of diffusion is to spread into a balanced amount. Simple diffusion is "going down the gradient", meaning it just spreads to the lower area VS. active diffusion, where its going up the gradient. Think of it as going uphill and downhill. which one is easier? going uphill or down hill? simple diffusion doesnt require ATP, but active diffusion does.