Advantage and disadvantage
When a player has advantage on a roll, they roll an extra die and discard the lowest. On most moves, that means rolling 3d6 and totaling the two highest dice.
When a player has disadvantage on a roll, they roll an extra die and discard the highest. On most moves, that means rolling 3d6 and totaling the two lowest dice.
When a player has both advantage and disadvantage on a roll, they cancel each other out.
See Special moves for a full explanation of advantage and disadvantage.