Sometimes a special ability or spell tells you that you have 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 lower of the two rolls. For example, if you have disadvantage and roll a 17 and a 5, you use the 5.
If multiple situations affect a roll and each one imposes disadvantage on it, you don’t roll more than one additional d20. If two situations impose disadvantage, for example, you still roll only 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 disadvantage and something in the game, you can reroll or replace only one of the dice. You choose which one.
You usually gain or impose disadvantage through the use of special abilities, actions, or spells, or even wearing certain types of armor. The DM can also decide that circumstances influence a roll in one direction or the other and impose disadvantage as a result.