Imagine a container with two chambers separated by a semi-permeable, i.e. semi-permeable membrane. In chambers 1 and 2, there is a liquid with dissolved particles, but the concentration of these particles in chamber 1 is higher than in chamber 2. As a result of the higher concentration, an OSMOTIC PRESSURE is now generated in chamber 1, which drives the liquid from chamber 2 to diffuse through the membrane. However, the dividing wall is insurmountable for the dissolved particles of the liquid. As a result of the fact that only liquid but no further particles enter chamber 1, the particle concentration in this chamber can be diluted and the pressure can be reduced or equalized. By the way, the process of liquid migrating through the membrane towards a more concentrated solution is known as osmosis.