Advantage and Disadvantage are bonuses that can be applied to any ability or skill check, save, or damage roll. Sometimes an effect of a character ability grants you advantage or an opponent disadvantage on specific types of checks. Other times, a creature may have advantage on a check due to circumstances that favor them, such as during a surprise round.
When you have advantage on a check, you roll twice and take whichever result is higher. When you have disadvantage, you roll twice and take whichever result is lower.
In mRPG, advantage or disadvantage on a given modifier also affects every other type of check that depends on that modifier. For instance, having advantage on Dexterity in a given situation also grants advantage on all dexterity-based skill checks, and on reflex saves, in that same situation. This effect does not work in reverse.