Advantage, Disadvantage, and Expertise
There are two ways that a creature's chances of success can be increased or decreased.
Advantage & Disadvantage
Sometimes a circumstance grants advantage or disadvantage on an ability check, a, or . When that happens, you roll two twenty-sided dice instead of one. For advantage, you use the higher of the two rolls. For disadvantage, you use the lower of the two rolls.
Some class features or abilities grant you an expertise die for an attack roll or saving throw, or in a specific skill or tool proficiency. When you make a d20 roll with which you have gained an expertise die, roll 1d4 and add the number rolled to the result of your check.
You can never roll more than one expertise die on the same roll. If another class feature or situation grants an expertise die that applies to the same roll, you don’t gain another die; instead, the size of the expertise die increases for that check, from 1d4 to 1d6, or 1d6 to 1d8. If you have a 1d8 expertise die on a check, further expertise dice have no effect.
If you have advantage or disadvantage at the same time as an expertise die, only the d20 is rolled twice, not the expertise die.
While advantage is most commonly used to represent circumstantial factors affecting a situation, expertise dice represents the particular training a character takes into the adventure.