RO = Purified by Reverse Osmosis
It basically blank slate water that you can build a mineral profile off of. I think the reason it is bad for yeast it that the water contains basically no minerals so all of the minerals inside of a yeast cell rush out to try to balance out the water inside their bodies with the water they are swimming in. Think your fingers in bath water for too long, the way they shrink up. Same idea just on a more deadly level.