From The Aquarium Wiki
A method of purifying water, often abbreviated in fishkeeping terms to RO (or R/O). RO water has all the minerals taken out of it via Osmotic pressure so it becomes very pure and soft with a ph of 7.
RO water is typically used to supply water for coral or reef aquariums. Though it is also used to supply water for freshwater Discus Cichlids as these animals like to be kept in very soft water. For use with freshwater fish it needs minerals adding back into it before it's safe for use.