Diffusion is the tendency for particles of any kind to spread out spontaneously from an area of high concentration to an area of low concentration. At equilibrium, molecules continue to diffuse back and forth, but there is no net change in concentration anywhere.
Passive transport is an extremely important way for small molecules to get into and out of cells.
For example, O2 moves into red blood cells and CO2 moves out of these cells by this process in the lungs.