Someone told you it was RO that came from an AC condensation drain?
Excuse me for a moment...
OK. Sorry.
RO means Reverse Osmosis. Reverse Osmosis pushes water through a membrane under high pressure to remove contaminants. What comes out of an air conditioner is condensation. You ever get yourself a nice glass of ice water and you get the condensation on the outside of the glass that starts dripping onto the table? That's what comes out of the AC units. Same thing.
Can you use it? Sure. Will hit hurt anything? Nope. Is it as good as RO water? Not on your life.
But RO comes with it's own disadvantages as well. Although it is almost always perfectly PH'd at 6.5, there are no nutrients in RO water at all. It is PURE water. So when you water with RO, you are literally giving your plant ZERO nutrients. But you're also giving it ZERO contaminants, which is the entire point of RO water: Your plant gets ONLY what YOU give it.
I run a 5 stage RO/DI unit that produces, quite literally, surgical grade water. You can get one for around 140 dollars from The Filter Guys:
http://www.thefilterguys.biz/ro_di_systems.htm