Even water treated by an approved municipal water system can lose it's goodness. Tap water is often marred by minerals, organic matter or chlorine. Reverse Osmosis is a process that filters such things out of water.
By using pressure to force a solution through a membrane, reverse osmosis retains the solute (dissolved solids) on one side and allows the treated solvent (water) to pass to the other side.
The membranes used for reverse osmosis water systems have a dense barrier designed to allow only water to pass through while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane.