What is reverse osmosis exactly? How does this save the planet?

Question

What is reverse osmosis exactly? How does this save the planet?

Answer 1

What the Geek said plus the fact that we can use it to get fresh water from polluted water or sea water.   We need clean water to live and irrigate our crops.   This will at least save part of the planet or delay its death or destruction.

Answer 2

I don't know how to answer the second question, but the answer to the first question is simple.  

If you have a semipermeable membrane separating two solutions of water, one have a higher concentration of stuff dissolved in it than the other, then assuming that whatever's dissolved in the water cannot pass through the membrane, water molecules will move into the solution with the higher concentration.  This process is called osmosis and require no input of energy to accomplish.  If we have the same semipermeable membrane, we can expend energy and force the water into the less concentrated side, thereby reversing the direction of osmosis.  Repeating this process a couple of times will yield very clean water.

Edit:

The reason I said I don't know how to answer the second part is because the process of reverse osmosis requires energy.  Right now, over 90% of our energy comes from fossil fuels.  The only way that reverse osmosis, or any other water purification method, could have no impact on the environment (and  even possibly contribute to helping it) is if the energy is provided by green sources such as solar, wind, hydroelectric, geothermal, or even from tides and waves.