Reverse Osmosis. It's a method of purifying the water.
Here's a link that explains the process:
http://science.howstuffworks.com/question29.htm
People also used distilled water, and there's kind of a raging debate going on over which is better that fires up now and then. Distilled water is water that's been boiled and then recondensed as steam. It gets rid of all the salts, bacteria, and heavy metals, but not other stuff.
