Disadvantage

Disadvantage reflects negative circumstances on a D20 Test (an ability check, saving throw, or attack roll), making you roll worse than normal.

Roll two d20s when making the affected roll and use the lower of the two results, then add your modifiers as usual. For example, rolling an 18 and a 3 means you use the 3.
It doesn't stack. Multiple sources of Disadvantage still mean only two d20s - there's no further penalty.
Advantage and Disadvantage cancel out. If a roll has both, you have neither and roll a single d20, no matter how many sources impose each.
Rerolls affect one die only. If an effect (such as Heroic Inspiration) lets you reroll or replace the d20, you change only one of the two dice - your choice - not both.

Disadvantage applies only to the specific roll the source names; it carries no other penalty.