It's a way of purifying water, water is forced through a membrane at high pressure, generally required for Marine tanks and exceptional circumstances in freshwater setups.
If your keeping and trying to breed some sensitive south american fish(and others than need soft water) like wild discus, apistogramma ect then RO is very good to have. Generally I wouldnt bother with RO unless I had a marine system- the RO units are fairly expensive to buy and maintain.