Advantage: throw one more d20, and keep the highest result. Disadvantage: throw one more d20, and keep the lowest result.
If you have both an Advantage and a Disavantage on the same roll then, thow a single d20.
Mulitple Advantages can stack, if you have double Advantage on a roll: throw 3d20 and keep the highest result. The same is true for multiple Disadvantages.