Milli-Q water is water sterilized via a Milli-Q machine. It is not necessarily RNAse-free, however. For that, you'll want Sterile Water.

When using one of the Milli-Q machines, first turn on the water flow, then wait for a few seconds. The water that comes out at first is not yet sterile, so you need to wait for the tube to clean itself out before pouring any water into your experiment.