dasaphrota - RO/DI can be confusing to some, so be sure to ask questions when they come up. IMO, RO/DI water is well worth the expense of the unit (especially when my tap water is a TDS in the high 400s).
I have no idea what my TDS reading is, I'm hoping it'll help me maintain decent nitrate levels as my tap water is in the nieghborhod of 20ppm so no matter what I do for water changes I can't get them down in my tank.
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.