What this guy is referring to is he lives in a hard water area, and uses rock salt to soften it - whether in the water tank or after I'm not sure, but theres nothing wrong with that. It just means the water out of the tap is soft water.
In fish keeping you'll hear hard and soft water referred to often - neither one is good or bad. Hard water has a high pH and soft water a low pH.
Different fish prefer hard or soft water - though most can adjust. If you look up any particular fish, most articles will tell you what pH range that fish prefers.
To find out what your water is like, either take your tap water in to your local fish shop (LFS) and ask them to test it for pH and make a note of what the number is. Or better still, buy a testing kit yourself - personally I think a testing kit that tests Ammonia, Nitrite, Nitrate and pH is essential for anyone who wants to keep their fish alive. If you get a testing kit buy one that uses liquid drops - they're more accurate and reliable than the test strips.
If the pH of your tap water is softer than the what the fish you intend to keep is, post up on here. Some fish will tolerate well outside of the recommended pH range for that fish, and its better to put them in a less suitable (but steady) pH than add products that will change the pH up or down - fluctuations are the bigger problem.
Hope I've explained that ok, if you're not clear on it post here again and someone will try to explain it better.