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.