For pool purposes, in layman's terms, RO is a type of very fine filtering performed via a mobile unit that essentially filters your water onsite through a special series of membranes designed to remove things such as cya and ch from the water. A lot of water is "wasted" in the process, but it still conserves more water than dumping and refilling a pool that has high concentration of cya or ch. it is an ideal solution in drought areas with water restrictions. The water cannot have algae, etc., or the filtering will not be productive.[1]
Reverse osmosis removes all (well most) of the dissolved solids from the water. That is everything: chlorine, TA, CH, CYA, Borates, Salt, everything.[2]
The equipment is specialized and pretty expensive for a large scale treatment like a pool so the service is not an option for most of the country. Usually only available where there is big demand due to higher temps, year round pool use or exceptionally hard water. For most of the country drain and refill is the only option.
