Advantage

For a comprehensive summary of the mechanics behind all rolls and modifiers, see: Die Rolls

Advantage and Disadvantage are a gameplay mechanic that can greatly affect the success of dice rolls. They can apply to Attack Rolls, Saving Throws, and Ability Checks, but not to Damage Rolls.

Advantage

When you roll with Advantage, you perform the roll twice, and use the higher result. You can only have Advantage once, in other words, you never roll more than twice due to Advantage.

Example: You roll two  d20 for an Attack Roll, because you have Advantage. The results are 16 and 4. Your effective result is 16.

Advantage is negated by Disadvantage. The number of Advantages and Disadvantages you have is irrelevant. Even if you have three sources of Advantage, a single source of Disadvantage will negate it.

Disadvantage

When you roll with Disadvantage, you perform the roll twice, and use the lower result. You can only have Disadvantage once, in other words, you never roll more than twice due to Disadvantage.

Example: You roll two  d20 for an Attack Roll, because you have Disadvantage. The results are 16 and 4. Your effective result is 4.

Disadvantage is negated by Advantage. The number of Advantages and Disadvantages you have is irrelevant. Even if you have three sources of Disadvantage, a single source of Advantage will negate it.