I believe it's advantage and disadvantage. So you roll 2d6 and take the best or worst of the two dice. So if you roll d6/a and get a 3 and a 5, you would take the 5. But if you were rolling d6/d you would have to take the 3 instead.
Correct me if I'm wrong 😂 I've only read the rules a few times and haven't played it yet.