Resistance to Magic (or, as it is called in Drakensang's character screen: Resist Magic) determines whether or not a hero is easy or difficult to enchant, whereby Willpower, a robust Constitution and an alert mind ensure that the latter is more likely.
Resistance to Magic is relevant if your hero tries to dominate another character by means of spells (most likely a Domination type spell). In those situations the Arcane Test that is taken for this spell, will incur a Difficulty Increase that equals the RM value of the spell target.
Calculation[]
(Courage + Cleverness + Constitution) / 5 +/- Character modifications