What does "RO" in RO water mean?

Reverse Osmosis, I think. I don't know enough about it to explain it.

Yep...

RO = Reverse Osmosis

and

RO/DI = Reverse Osmosis/De-ionized

Reverse Osmosis filtration works by having a membrane that permits H2O to passively go through--ie, osmosis is the process of a substance going from high concentration to an area of low concentration--while blocking most of the other molecules (calcium, potassium, phosphates, nitrates, etc.)

Slight clarification - osmosis involves a membrane which allows water molecules to go through while blocking dissolved mineral ions. Reverse osmosis requires pressure to force the water molcules through, as it is the reverse path from conventional osmosis.

In conventional osmosis, water moves across the membrane from a higher concentration of water to a lower concentration of water (example: FW on one side, SW on the other - water moves from the FW side to the SW side in increase the water concentration on the SW side, eventually it will equalize the two).

In RO, we are forcing water to move from the lower concentration of water (i.e., water + minerals to water with fewer minerals), so it is the reverse of normal. We are increasing the water concentration by the use of pressure.

HTH

Thanks for the clarification, RTR!