Diffusion is the random molecular motion of molecules or atoms due to their kinetic energy. Generally things tend to diffuse from an area of higher concentration, pressure, or temperature towards an area of lower concentration, pressure, and temperature. In living organisms, concentration is generally the most important factor affecting the direction of diffusion, but temperature is most important in determining the rate of diffusion.
Temperature is a measure of how much molecular motion there is.
Since diffusion is molecular motion, if there is more molecular motion, there is also more diffusion. All chemical reactions that happen in cells depend on the diffusion of the molecules and enzymes involved in the reaction so that they collide with each other in just the right way for the reaction to occur. Diffusion is also how small molecules like water and oxygen get into and out of cells through the membrane. When a larger molecule needs to be carried through a membrane it must first diffuse to the membrane and bind to a specific carrier molecule which will carry it across the membrane. The absorption of nutrients from the digestive system depends on diffusion.