Its just tap water run through salt pellets. It lowers the water hardness. You should use RO water, or some other equivalent you know is free of impurities.
as lobster said the impurities will remain in the water, you need ro/di water.
soft water is good for cleaning dishes and taking showers... thats about it
Well the truth is that the water runs thru Resin in the media tank and goes thru a there is an ion exchange. Tis removes Calcium, Magnisium, and Iron. These minerals well ruin pipes,hot water heater,clothing,any water using machinery. Also you can cut your soap use in 1/2. This process will also increase your membrane life about 2 years. I do reccomend a softener and an RO/DI.
I got a "Rainsoft" brand water softner last year that came in a package deal with a 50 gpd RO unit. (I got installer to leave me extra tubing and I bought float at LFS from Spectrapure and plumbed up RO unit to sump for auto-top-off. There may be better RO units out there, but to get a package that takes all the junk out of my water and a RO unit for the fish and drinking water made me happy.