Anyways, DRN is discussed in the manual on page 5 as one of the fundamental game mechanics. (Jokingly) RTFM (Read The [censored] Manual).
I understand where you're coming from, HoneyBadger, and I also understand why IndyPendant didn't feel the need to expound. "Probabilities in Dominions" isn't the most exciting manual section, and easy to overlook.
A DRN is a Dominions Random Number. This is a roll of two six-sided dice, open-ended (roll six, re-roll that die, add new number to previous total).
As a point of clarification - technically, if DRN shows up in the formula anywhere, it is always open-ended roll. If "DRN" does not show up, and you only see 2d6 (two six-sided die rolls), then its not open-ended.