Dasaphrota,
TDS = Total Dissolved Solids - Just a way to help give you an idea of how "clean", for lack of a better word right now, your water begins. As Nikki (NaH20) mentioned, her tap water begins with a high level of Dissolved Solids. I don't know for sure, but I would venture that after her RO/DI unit, her Dissolved Solids are now down to less than 10 for her! WONDERFUL improvement for our tanks!!!
As for your Nitrate levels... the use of a RO/DI unit is exactly why many of us have gone to them! The RO/DI removes the Nitrates, Ammonia, Coper, and many other impurities from our tap water, making it a much more "Friendly" water for our Reefs. The output from your RO/DI unit should have ZERO Nitrates, Ammonia, Coper... just to name a few! When that ZERO water is what your using for make-up water, and water changes... just think how much healthier this is for our Reefs! *Smile*
Hopefully, in a very simplistic way, this helps you understand what an RO/DI unit is doing.