Final answer:
Extra Attack is a feature in Dungeons & Dragons that allows a barbarian to attack twice instead of once when they take the Attack action on their turn, starting at the 5th level.
Step-by-step explanation:
In the context of the game Dungeons & Dragons, which is sometimes explored in AP courses on storytelling and gaming, Extra Attack is a feature that grants a barbarian the ability to attack more than once on their turn. This means a barbarian, upon reaching a certain level, can make additional attacks in one action. Specifically, at 5th level, barbarians gain the Extra Attack feature, allowing them to make two attacks, instead of one, whenever they take the Attack action on their turn. This significantly increases a barbarian's potential damage output during combat.