Advantage is a simple system a Game Master can use to reward players for using smart tactics or having an edge in a situation - or simply to add some role playing flavor. When a Game Master decides that a player should be extra likely to succeed at a dice roll — combat rolls, saving throws, and ability checks — they can declare that the roll be made with advantage.

Rolling with advantage means the player rolls two 20-sided dice instead of one and uses the higher number.

Attacking a monster from a secret hiding spot?
Roll with advantage!

Disadvantage is the exact opposite; the player rolls two 20-sided dice instead of one and uses the lower number.

Attacking a monster while blinded in darkness?
Roll with disadvantage.

Remember advantage and disadvantage don’t stack. A player can never have double advantage. Multiple advantages only add up to a single advantage roll. But, they do cancel each other out. If a player has advantage and disadvantage on the same roll, they will cancel each other out, and the roll is made normally.