“A shield that increases defense by 2.” —In-game description, New Mystery of the Emblem
The Dracoshield (竜の盾 Ryū no tate, Dragonshield in the Japanese version) is a Stat booster that recurs across the Fire Emblem Series. When used, the Dracoshield permanently raises its user's Defense by 2. In general, the Dracoshield is a rather uncommon item, and each acquisition of the item should be used on units suffering from low Defense growths.
In Genealogy of the Holy War, the Dracoshield is substituted by the Shield Ring (シールドリング Shīrudo ringu), an item that must be placed in the inventory of any unit in order for its Defense-boosting property to take effect. The Dracoshield is similarly substituted by the Shield Ring in Thracia 776, although this particular incarnation of the ring acts as an expendable item that permanently boosts its user's Defense by 2 when used.
In TearRing Saga: Utna Heroes Saga, the Dracoshield's equivalent is a potion that is simply known as the Defence Plus (DEFプラス DEF Purasu), permanently raising its user's Defense by 2 when used.
While it may be tempting to use a Dracoshield on units with extremely low durability, such as Clerics, two points of defense will not provide much additional functionality to units that are already ill-suited for the frontlines. Dracoshields should be used on frontline units that are expected to take a lot of abuse, as the two points of damage reduction will really build up over several battles. Units with high evasion and weak but not terrible defense can benefit from Dracoshields as well.