Advantage

Sometimes a special ability or spell tells you that you have advantage or Disadvantage on an ability check, a saving throw or an Attack roll.
When that happens, you roll a second d20 when you make the roll.
Use the higher of the two rolls if you have advantage. For example, if you have advantage and roll a 17 and a 5, you use the 17.
If multiple situations affect a roll and each one grants advantage, you don't roll more than one additional d20.
If circumstances cause a roll to have both advantage and disadvantage, you are considered to have neither of them, and you roll one d20. This is true even if multiple circumstances impose disadvantage and only one grants advantage or vice versa. In such a situation, you have neither advantage nor disadvantage.
When you have advantage and something in the game lets you reroll the d20, you can reroll only one of the dice. You choose which one.
You usually gain advantage through the use of special abilities, actions, or spells. The GM can also decide that circumstances influence a roll in one direction or the other and grant advantage as a result.