Resistance (魔防 Mabō lit. Magic Defense) is the Fire Emblem statistic that defines a unit's ability to defend against magical attacks. The higher a unit's resistance, the less damage they'll recieve from magical attacks. Magic users such as Mages and Monks tend to have high resistance.
In the first two Fire Emblem games, resistance is considered a special stat, as it cannot increase via levelling up, and can only be increased through Talismans. Additionally, in the first game, only Gotoh starts out with a resistance stat above 0. In Fire Emblem: Mystery of the Emblem, every playable character has a 3% resistance growth rate, and from Fire Emblem: Genealogy of the Holy War onward, resistance has become a normal stat, excluding Fire Emblem Echoes: Shadows of Valentia, where playable characters have a 1-8% resistance growth rate, with the exception of any potential Dread Fighters, who all have growth rates of 0%.
Non-magic units usually have very low resistance, much like magic units have very low defense. In most games resistance can be boosted by two points with a Talisman. However, Pure Water and a Barrier staff can temporarily increase resistance by 7 points. Ninis' Grace can be used by Ninian or Nils in Fire Emblem: The Blazing Blade, to increase resistance by 10 for 1 turn.