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 will 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 became 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.