Strength (力 Chikara) is a character stat that affects the amount of damage that the character inflicts when using a physical (non-magic) attack weapon.
Strength is one of the two major contributing factors to Attack Power, the other being the Might of the weapon being used. The higher the strength a unit possesses, the more damage that unit will inflict, except for Crossbows.
In Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn, Strength is also used to calculate Attack Speed. The higher a unit's strength, the heavier the weapon can be before the unit suffers an attack speed penalty. Units like Fighters, Warriors, Pirates, Berserkers, Knights, Generals, etc. usually have large amounts of strength. Strength is very good at slicing through an enemy's Defense. Units such as Thieves, Pegasus Knights, Myrmidons and Swordmasters usually have low strength. Magic units usually possess lower strength because they use magic, instead.