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.
I've never used this service, but essentially, the supplier bring the equipment onsite, runs the process, then removes the trailer/truck to the next job.
For household drinking water and salt water aquariums, the R/O systems are much smaller and permanently installed, but likewise remove impurities from the water, and lower the TDS (total dissolved solids). In the realm of aquariums, this is important, especially if you're on well with iron In fact, too much iron will foul the membranes quickly and best practice for household use is to plumb the RO after the water softener.
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.
Last year with the drought in norcal they wanted $600 to filter a pool up to $20k. Cost of water for a drain and refill was $150 so really wasnt cost effective. Sanitary district will require you to apply for a permit in some areas and restrict gpm flows into the drains so as to not inhibit ater treatment. No cost for the permit in my town but others may be different.
Ive also seen where people place a tarp vertically in the pool and drain from one side so as to not waste as much water when doing partial drains to get down CYA levels.