The reverse osmosis membrane is composed of a support layer (thickness 50 micron) and an active layer with separation function (thickness 0.2 micron) and separates a solvent and a solute. When the solutions with concentration difference like salt water and fresh water are separated by the semi-permeable membrane, after a certain time passes, the water in the solution with the lower solute concentration moves to the solution with the higher concentration to make a difference in the level of the solutions.
This phenomenon is just called "Osmosis" and the solution level difference created at this time is the osmotic pressure.
On the contrary, if a pressure greater than the osmotic pressure is applied to the solution with the higher solute concentration, water moves from the solution with the higher concentration to the solution with the lower concentration, and the solute in the solution with the higher concentration cannot pass through the semi-permeable membrane, which further increases the solute concentration in the solution with the higher concentration. This phenomenon is called "Reverse Osmosis" and the facilities which this principle is applied to are called ""Reverse Osmosis Facilities".