It is said to be the biological process of movement of the biochemical entities across the cell membranes and tissues.
Movement of ions and several other molecular within the cells takes place along the concentration gradient, without any external energy.
Some examples are:-
Reabsorption of nutrients in the intestines occurs by separating them from the solid waste and transporting the nutrients through the intestinal membrane into the bloodstream by simple diffusion.
When a raisin is soaked in water, then the water moves inside the raisin by the process of osmosis and gets swelled up.