Diffusion means movement of particles through material.  Diffusion is important in physics, chemistry and biology.  Diffusion is affected by temperature, the size (mass) of the particles that are moving, and the thickness (viscosity) of the environment in which they are moving.  Temperature is a measure of kinetic energy.  Diffusion is affected by this kinetic energy.  Basically, increasing the temperature increases the molecules speed (kinetic energy) and decreasing it decreases the speed.

Generally, the heavier the particle the more it interacts with its environment and the more the temperature matters.  Higher temperatures usually result in higher diffusion.