Advantage and Disadvantage
Sometimes, you have an edge in a situation. A special equipment might grant you a burst of strength for a check, an enemy might be unaware of your attack, or your protective gear might make it easier to avoid danger. In such situations, you have advantage.
Other times, the odds are against you. Your enemies might interfere with your actions, some demoralizing effect might thwart your concentration, or you might try to attack while hanging onto a cliff face. In such situations, you have disadvantage.
You usually gain advantage or disadvantage through the use of special abilities or because of particular conditions. Your GM might also determine that circumstances are in your favor and grant you advantage, or that they are not in your favor and impose disadvantage. It is very rare to have permanent advantage on a given task.
If you have advantage or disadvantage on a check, attack roll or saving throw, you roll an additional d6 when making that roll. You use the highest 3 dice to determine your result if you have advantage and the lowest 3 if you have disadvantage.
Multiple Instances of Advantage and Disadvantage
If you have multiple instances of advantage or disadvantage (but not both), the GM must decide if they stack. If they do, roll an additional d6 for each instance of advantage or disadvantage. No matter how many dice you roll, you only total the highest or lowest 3. This is referred in the rules as having double, triple, etc. advantage or disadvantage.
If you have the same number of instances of advantage and disadvantage on the same roll, they cancel each other out for that roll.
If you have a different number of instances of advantage and disadvantage in the same roll, they cancel each other at the rate of one advantage per one disadvantage. The end result will be one or various instances of advantage or disadvantage, but not both.