Diffusion is the passive movement of particles from an area of high concentration to an an area of low concentration.
Particles go through the cell membrane from where there are more particles (high concentration) to where there are less particles (low concentration)
Particles move randomly so they go both ways, but the most movement tends to be away from the area of high concentration.
The rate of diffusion is affected by the concentration difference. Substances diffuse faster if there is a bigger concentration difference, if there are lots more on one side then there are more to move across.