Diffusion is the spreading out of one substance into another. But there's a bit more information we need to add to this definition to understand diffusion fully.
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.
Think you have got this idea? Let's see some scientific examples in the following questions!