Diffusion is the movement of something from where there is a lot of it to where there is less. For example, if someone is baking cookies, that pleasant smell shows up near the stove first. Little by little, it spreads throughout the whole room. The molecules with the cookie aroma "diffuse," moving from around the stove to other areas.
Diffusion occurs in liquids as well. When a drop of food coloring is placed in a cup of water, it starts to move throughout the water. It keeps going until it is spread evenly in the entire cup. Diffusion happens in gases, like air and liquids, like water, because the particles move so freely.
Gases diffuse quickly because their molecules are so spread out. Liquid molecules are closer together, so diffusion is a bit slower.
Solids do not diffuse because their molecules move very little. They only vibrate in place. However, a gas may diffuse through a solid. For instance, a balloon gradually deflates, getting smaller and smaller. This is because the air diffuses through the balloon. Air molecules move from where they are packed lightly, or concentrated, inside the balloon to where they are more spread out, or less concentrated, in the open air.