RO= reverse osmosis. Ill explain in brief. Osmosis is the process by which a solvent moves from a low concentration solution to a higher concentration solution when separated by a semipermiable membrane, (teflon in most cases). ie for eg is sea water and pure water is separated by a membrane in a vessel, the pure water will move towards the salty water to equal the concentration of salt. In reverse osmosis, this process is reversed by applying pressure which is more than the osmotic pressure to salt water so that pure water from salt water will move to the pure water compartment.