A tube has a semi-permeable membrane at the center with two solutions on either side. The solution on the left has a higher concentration than the one on the right. Thus the solvent will move from the righthand side to the lefthand side to get to the solution of lower free energy. This will lead to a dilution of the solution that was initially more concentrated and a concentration of the solution that was initially more dilute. The flow will continue until the two sides are equal in free energy. The movement of solvent from one side to the other, will lead to a height difference between the two sides. This height can be converted into a pressure.