RODI is reverse-osmosis / deionised water — tap water purified of the nitrate, phosphate, silicates and metals that feed algae and harm inverts — and it's what you mix your salt with.
Starting with impure tap water imports exactly the nutrients and contaminants you spend the rest of the hobby fighting. An RODI unit at home, or bought RODI/saltwater, is one of the highest-value beginner investments.