Sometimes, certain conditions are favorable or unfavorable for your character. These situations are specific, such as a creature being prone granting Advantage to melee attacks against that creature. Advantage means you roll the dice twice, and take the higher of the two, while Disadvantage means you roll twice, and take the lower of the two.

If you have advantage and disadvantage simultaneously, the roll is made normally, neither at disadvantage or advantage.

You cannot stack advantage, and having more instances of advantage or disadvantage does not affect the situation. You either roll normally, or at advantage or disadvantage.