diffusion is the net movement of a substance, this is generally from a area of high concentration to low concentration, the particles will collide or 'bounce' off one another and by random movement, will eventually spread out till they are even.
Diffusion is the movement of particles from a high concentration of said particle to a lower concentration, thus balancing out the difference. The difference between two concentrations is called the concentration gradient (So a high concentration would be a higher difference). If there is a higher concentration gradient, the rate of diffusion will also increase.
