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. R/O is a good way to recover water that would otherwise be dumped due to high CYA or Calcium Hardness and ergo is environmentally astute, especially where water conservation is an issue.
Water must be algae free to do a RO treatment.
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. Southern California, Arizona, and Dallas are known to have Reverse Osmosis services.
All chemical levels need to be checked and the water has to be rebalanced after a reverse osmosis treatment is completed on the water.