Diffusion is the net movement of fluid from area of higher concentration to lower concentration, resulting from the random movement.
From example, if you drop a red ink in a cup of water, the ink will slowly spread among the water, making the water red. This is because the red ink is very concentrated, so it moves to the other part of water to lower the ink concentration. When it reaches the point where whole cup of water is red, it reached equilibrium and there's no more net movement
