Definition: Reverse Osmosis is the process of removing salt from seawater. Seawater is different from, for example, river water (fresh water) which doesn't have salt in it. To make seawater drinkable we need to process it and remove the salt.
This can be done in one of two ways. The first is distillation - the setup is similar to one you might use in the lab, but this is done on an industrial scale where glass tubes are replaced with vast towers. The seawater is heated to the point where it turns to steam which rises into the tower where it condenses and is then collected. This same process can be carried out using a different design where the water cools and condenses on a 45-degree pane and the runoff is collected at the bottom. A number of other configurations can be used but the process is the same, ie, to end up with water that has no salt in it.
Second process is filtering the salt out of the seawater - the water passes through the filter trapping the salt crystals on the other side. As this process is continued the salt concentration in the remaining water becomes higher.