Advantage is a dice mechanic that increases (in a roll-over game, which is more commonly) or decreases (in a roll-under game, less often) the likely result of a dice roll without affecting the range of possible results. Advantage is applied when a character is in a beneficial situation that should increase their odds of success, and is an alternative to flat modifiers. A similar mechanic that reduces a character's odds of success is called disadvantage. Advantage was popularised by its use in Dungeons & Dragons 5th edition.
Advantage works by rolling an additional die of the same type used in a standard roll, then ignoring the result from the lowest scoring die. That is, if an ordinary resolution roll would use a single d20 (as in Dungeons & Dragons), then rolling with advantage means rolling 2d20 and keeping only the higher result. If an ordinary roll would use 2d6 (as in many Powered by the Apocalypse games), then rolling with advantage means rolling 3d6 and keeping only the highest 2 dice.