Depending on where you live and where your water comes from, there could be various chemicals added to the water to help purify it. While it may not harm us, it can and does harm the fish. That is why it is not recomended to use straight tap water for your aquarium. Do a search on here for reverse osmosis and you will find plenty of threads on the subject.
P.S. Dont feel stupid. I didnt know what RODI was when I first set up my aquarium, most of us didnt.