Through natural quirk or potent artefact, some warriors have an innate resistance to magical attack.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
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A model with Magic Resistance gains a bonus to dispel enemy spells directly targeting it (not including templates that are placed on top of it) as well as a Ward save against damage caused by spells. This bonus is based on the number shown in brackets after the Magic Resistance special rule. So, Magic Resistance (1) would give a +1 bonus to dispel rolls and a 6+ Ward save, Magic Resistance (2) would give a +2 bonus to dispel rolls and a 5+ Ward save, and Magic Resistance (3) would give a +3 bonus to dispel rolls and a 4+ Ward save, etc. Note that Magical Resistance does not offer protection against Miscasts.
If a character with Magic Resistance joins a unit, all models in the unit benefit from the Magic Resistance as long as they are part of the unit. If a model or unit has two sets of Magic Resistance, the two combine to a maximum of Magic Resistance (3), unless specified.