Fact:

In simple terms, it is the steady movement of particles from regions where there are many of them to regions where there are a  few of them.  It is a natural trend for things to spread out down a Concentration Gradient. See diagram below. 

In scientific terms, DIFFUSION is the MOVEMENT OF PARTICLES from a region of HIGH CONCENTRATION to a region of LOW CONCENTRATION. 

Diffusion is a passive process, it does  not require any energy for it to happen.

For a leaf to carry out PHOTOSYNTHESIS, it requires carbon dioxide gas.  There is more carbon dioxide outside the then inside.  This has to get inside the leaf.  It does this by DIFFUSION through minute little pores beneath the leaf called STOMATA.  

Similarly, OXYGEN and WATER VAPOUR leaves the leaf using the same process of diffusion.