Diffusion occurs when a substance moves from an area of high concentration to an area of low concentration. Concentration can be described as how much of a substance there is in relation to the other substance molecules in the area it occupies. Molecules naturally disperse because they want an environment that is stable. Concentration can be illustrated by thinking about smoke inside of a house from a fire. The smoke will build up inside the house (making it highly concentrated) but if a window is opened the smoke will pour out to the outdoors (an area that is low concentration).
An example of diffusion can be seem when you add food coloring to a glass of water. The first drop of food coloring in the water is highly concentrated because there are a lot of food coloring molecules within that drop. As time passes by the food coloring will diffuse throughout the water making the entire glass colored. The food coloring has gone through diffusion, moving from an area of high concentration (the single dark drop of coloring) to areas of low concentration throughout the water.