You will notice that the tanks water level is going to decrease slightly everyday due to evaporation. You must replenish this evaporation with fresh water, NOT SALT WATER. Salt will not evaporate with the water, and adding more salt water will make the salinity begin to climb. Only add more salt water when you remove salt water, during things such as water changes or acclimation.
Also, you do not want to use fresh tap water to top off the tank due to contaminants and other things that you typically would not want in your salt water tank. Therefore, an RODI unit sends the waters through various filters and spits out the purest water you could get your hands on. All you do is hook it up to your garden hose or sink faucet. I personally hook it up to my garden hose, and fill up two 55 gallon drums.
If you are new to the hobby, I highly recommend going through the sticky threads at the top of this forum, and start googling every question that pops in your head, as Google is a great way to search the reef central threads as they always pop up on the first page.