To Drop, in the context of the Fire Emblem franchise, is the act of an enemy unit dropping an item upon their defeat.
Overview
Enemies may have inventories that allow the player to take certain Weapons or Items. Upon their defeat, such enemies will drop these weapons and items and the attacking character automatically claims the item. Players can see which weapons and items will be dropped by an enemy if it is highlighted in Green text in most instances. Since most units have a limited inventory space when on the battlefield, if a unit picks up a dropped weapon/item, the player has the option to send another item to the Convoy or send the item itself to it.
Thieves commonly are utilized on maps with Chests, which they can steal from. Once they do so, these units usually attempt to flee battle as soon as possible and take away their spoils. Players may intercept them to claim the stolen goods. Thieves can occasionally steal multiple items and they will drop all of these items once defeated, allowing the player to use them as a free means to plunder the chests.
Player characters technically drop their inventory if they are defeated in combat in Classic Mode, allowing the player to reclaim at least their items from their lost unit automatically, placing them in the Convoy.