Diffusion is the movement of particles of a substance from an area of high concentration (where there are a lot of them) to an area of low concentration (where there are fewer of them).
High concentration = A large number of particles of one substance in a specific volume of another substance.
Low concentration = A small number of particles of one substance in a specific volume of another substance.
As you can see in the picture below, there is an area where there is a high concentration of dogs (where they are all grouped together). Diffusion is when a substance - in this case dogs! - moves from an area where they are in high concentration to an area where there is a lower concentration of them - so the dogs will spread out into the flowers.
As you can see, the dogs have now spread out as far as possible - diffused - so they are now at a lower concentration.
Diffusion will take place differently in different substances. For example, diffusion cannot really take place in solids as the particles in a solid cannot move very much. But diffusion takes place very quickly through gases, and less quickly in liquids, because the particles of a gas are spread out and move fast - faster than particles in a liquid.
Think you have got this idea? Let's see some scientific examples in the following questions!