“An item that increases strength by 2.” —In-game description, Shadow Dragon and New Mystery of the Emblem
The Power Ring (パワーリング Pawā ringu) is a Stat booster that recurs across the Fire Emblem Series. When used, the Power Ring permanently increases its user's Strength by a set number of points that varies from game to game.
In the GBA titles of the series, the Power Ring is known as the Energy Ring (エナジーリング Enajī ringu), where it permanently increases its user's Strength or Magic (depending on the type of weapon that they wield) by 2 points.
Starting from Fire Emblem: Path of Radiance, the Power Ring is renamed to the Energy Drop (エナジーの雫 Enajī no shizuku), where it also permanently increases its user's Strength by 2 points when used.
The Power Ring/Energy Ring/Energy Drop is useful to virtually any unit that uses its Strength (or Magic, in the GBA games) to attack. It is particularly useful to low level units, as it will help them secure kills and thus gain more EXP, hopefully giving them a good chance to catch up to the rest of the army. It is also useful in the hands of units with high Speed stats or abnormally high Critical rates,as its +2 to Strength could translate into +4, +6, or even +12 damage over the course of a single fight.