The concept of Reverse Osmosis:
A solution has a natural osmotic pressure. If we have one concentrated solution and one dilute solution separated by a semipermeable membrane and we apply pressure to the concentrated side.
If the applied pressure is higher than the osmotic pressure, the solvent of the concentrated solution (which is water) will flow through the membrane to the dilute side, thus on concentrated side become more concentrated and has a higher osmotic pressure now.
If the applied pressure is equal to the solution's natural osmotic pressure, no flow will occure.
If the applied pressure is less than the natural osmotic pressure of the solution, there will be a solvent (i.e. water) flow from the dilute solution to the concentrated solution.
A natural osmosis is that the water flows to the concentrated side.
Reverse osmosis is a process whereby the natural phenomenon of osmosis is reversed by applying pressure to the concentrated side of the semipermeable membrane.