Reverse osmosis is a filtration method applied to the desalination of water. Osmosis is the passage of solutes through a semi-permeable membrane from a region of high solute concentration to a region of low solute concentration, until both regions have reached equal solute concentration. The energy that drives this process is called osmotic pressure. In reverse osmosis, this osmotic pressure has to be overcome to drive the solutes from a region of low to high concentration.
When reverse osmosis is used in a desalination plant, high pressure pumps provide the pressure to push the smaller water molecules through a membrane, while the larger salt molecules, as well as any bacteria, viruses and other contaminants are left behind.