The process of diffusion is the random movement of substances from a region of high concentration to a region of low concentration until no more concentration differences remain.
For example, consider what happens when you make a cup of tea. The tannin molecules (which give the tea its golden-brown color) are very concentrated in a tea bag and are absent from the hot water. As they start to dissolve, the molecules inside the bag begin to move faster and faster. One molecule bumps into another, then into a second and a third, and so on until gradually the molecules spread out, leave the vicinity of the tea bag and turn the water a marvelous golden color.
Slowly the tannin molecules have moved away from a region of high concentration near the tea bag and into the lower concentration of the surrounding water until eventually the tannin molecules are at equal concentrations throughout the liquid.
As the tannin molecules randomly disperse throughout the liquid they are obeying the second law of thermodynamics, and the amount of disorder in the universe is increased.