Osmosis is a natural process for which a more diluted solution passes spontaneously to a more concentrated solution by means of a semipermeable membrane.
The concept of reverse osmosis is simple, as it suffices to apply a pressure to a concentrated solution superior to the osmotic pressure in order to provoke an inverse flow in respect to the natural one, extracting the dissolved salts from concentrated solution, that creates a discharge flow; on the opposite part of the membrane we have a low salinity solution.