Some units have a natural resistance to magic, or they are protected from hostile magic by magical artefacts they possess. The number in the brackets indicates the maximum number of extra dice that may be rolled when trying to dispel each spell that is targeted against the magically resistant unit. These can either be used on their own against any spell targeted against the unit, or in combination with dispel dice from Wizards or other sources. For details of dispelling see the Magic section.
If a character with magic resistance joins a unit, the resistance affects the entire unit. In the same way, if a character that is not magic resistant joins a unit that is, the character will be protected by the unit's magic resistance, even if he is the only model affected by the spell.
Magic resistance is never cumulative, so if a magic resistant character joins a magic resistant unit, or a different magic resistance is bestowed on the unit by a spell or similar, never add the different magic resistances together, but use the best available.