Basically is super cleans up your water, leaving it very pure compared with plain tap. Kinda like the difference between regular water & softened water, that sorta the difference between softened water & RO water.
Here's a quickie RO tutorial from google.
"What impurities will reverse osmosis remove?
Reverse osmosis (RO) has become a common method for the treatment of household drinking water supplies. Effectiveness of RO units depends on initial levels of contamination and water pressure. RO treatment may be used to reduce the levels of:
Naturally occurring substances that cause water supplies to be unhealthy or unappealing (foul tastes, smells or colors).
Substances that have contaminated the water supply resulting in possible adverse health effects or decreased desirability.
RO systems are typically used to reduce the levels of total dissolved solids and suspended matter. The principal uses of reverse osmosis in Minnesota and the Dakotas are for the reduction of high levels of nitrate, sulfate, sodium and total dissolved solids.
RO units with carbon filters may also reduce the level of some SOCs (soluble organic compounds) like pesticides, dioxins and VOCs (volatile organic compounds like chloroform and petrochemicals). An RO unit alone may not be the best solution for these types of contaminants, but installing a properly design-ed RO unit to reduce the levels of other contaminants may provide a reduction in SOCs and VOCs. "