Advantage and Disadvantage - How to Play

Player's Handbook, D&D Basic Rules, D&D Player's Basic Rules

Sometimes an ability check, attack roll, or saving throw is modified by special situations called advantage and disadvantage. Advantage reflects the positive circumstances surrounding a d20 roll, while disadvantage reflects the opposite. When you have either advantage or disadvantage, you roll a second d20 when you make the roll. Use the higher of the two rolls if you have advantage, and use the lower roll if you have disadvantage. For example, if you have disadvantage and roll a 17 and a 5, you use the 5. If you instead have advantage and roll those numbers, you use the 17.

More detailed rules for advantage and disadvantage are presented in chapter 7.