Advantage
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'll never roll more than twice for a single attack or check 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. [Needs confirmation, because it is the same text like in Disadvantage]
Examples of situations that grant Advantage:
- Attacking an enemy that is under these conditions: Restrained, Prone, Sleeping, Entangled, Paralysed
- Being unseen by the enemy while making an attack
- Various spells and abilities that grant Advantage
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'll never roll more than twice for a single attack or check 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. [Needs confirmation, because it is the same text like in Advantage]
Examples of situations that grant Disadvantage:
- Trying to make a ranged attack against an enemy that is within 5ft of you
- Trying to attack an enemy obscured by darkness
- Various spells and abilities that grant Disadvantage
