When you make a roll with advantage, roll an extra die and discard the lowest result.
When you make a roll with disadvantage, roll an extra die and discard the highest result.
When you make a roll with both advantage and disadvantage, they cancel each other out.
If you have advantage/disadvantage on a damage roll, roll the main die twice and discard the lower/higher result. Then add any bonus dice that apply.
(For more information, see Player Moves.)