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. In Fire Emblems 1, 2, 3 and the GBA games, however, units who used magic did not have a strength stat at all. The magic stat, in this case, replaced strength altogether instead of being a separate stat alongside it.
Karel (in FE6) has the highest strength growth in the entire series, numbering at 130%. Others with exceedingly high strength growths (80% or above) include Manaketes like Fae or Myrrh, human characters such as Wolf and Sedgar (with the proper classes in Shadow Dragon), several characters in FE12 reclassed to Warriors (e.g. Luke, Sirius, Ymir), Walhart, some children of Donnel and generally every inheritor of Hezul holy blood.