Reverse Osmosis is the process of stripping all (well most anyways) of the dissolved salts out of water.
The water is run through a series of filters and membranes. Like above post stated, for me to make 1 gallon of ro water, i waste approx. 3 gallons.
This process will make water with a very very low ppm.
Well worth the cost, if you ask me, to know exactly what nutrients your plants are getting.