If we put a drop of ink into clear water, then even without stirring, the ink will slowly spread throughout the water. Behind this is the motion of the ink molecules associated with the temperature of the system. The motion of each molecule is purely random, but eventually the sum of many random steps will carry a sizeable number of the molecules far away from the location where we have put the ink drop in. Processes like this where random motions lead to a spreading-out of an ensemble of molecules or other entities are called diffusion.