He he he... its a good thing you disabled your rep polishpollack. I dont know if you can get negative rep but you'd be down there if you could judging by that post and another I've seen.

TDS/PPM - Total dissolved solids/parts per million.

This is the amount of other things in your water apart from the water itself. Sodium, Calcium, Magnesium etc. A TDS meter is a vital piece of equipment if you are growing in hydro. Without it, success is going to be near impossible.