RO water is reverse osmosis water frequently seen paired with the abreviation DI which has to do with ionization. This can be purchased from some large chain stores (wegmans near me sells it near me) or made through filtering (the way I do it). A typical filter will run you 100-350 dollars, but its worth it if you want to stay in the hobby for long.
The water you are adding every time you do water changes, fill the tank, top off ect has levels of iron, copper, and other harmful stuff in it usually depending on the water company you are serviced by.
RO/DI water filters hook up to your tap water faucet and filter the water to eliminate these trace elements (and believe me there is more than you think in your tap water). Using tap water isn't the end of the world (although it isn't great either), but RO/DI water will cause far less damage to your aquatic friends and help cut down on algae blooms.
Definitely look into a filter or find a place that sells RO or RO/DI water near you...