Rolling
Many moves tell you to "roll +STAT" (usually with a specific stat, like STR or INT, see Stats).
"Roll" means: roll two six-sided dice (2d6), and add them together.
"+STAT" means: find that stat in your PC's playbook and add it to the roll. Stats range from -1 to +3.
For example: I say that I elbow this guy in the face. We agree that I'm fighting in close quarters, triggering Clash. I roll 2d6 and add my PC's STR. If my STR is +1, my roll might look like this:
+ + +1 (STR) = 8
You'll sometimes have advantage, meaning: roll an extra die and discard the lowest. With the advantage of an extra die, the roll above might look like this:
+ + + +1 (STR) = 10
And sometimes, you'll have disadvantage: roll an extra die and discard the highest. With disadvantage, that roll might look like this:
+ + + +1 (STR) = 7
If you have advantage and disadvantage on the same roll, they cancel each other out. Roll normally (2d6, add them).
Advantage and disadvantage normally stem from a move or condition. The GM can apply them ad hoc if they deem a roll especially hard or easy, but they're encouraged not to. See Advantage/disadvantage.