It's in general. Many (but not all) monsters have Magic Resistance, which is listed as a percentile score (e.g. MR 40%).
Magic Resistance is checked whenever a spell is cast on a creature that has it, prior to making a saving throw. When that happens, the caster needs to roll percentile dice and score above the listed MR number for that monster. If the roll is equal to or less than that number, the spell fails to affect it completely (though, if I recall correctly, if it's an area-effect spell, it still potentially affects other creatures in the area). If the spellcaster does roll above the MR value, then the spell works normally, and a saving throw (if any) is made, etc.
One of the major differences between 2E Magic Resistance and 3E Spell Resistance is that the value of 2E MR is flat - it doesn't change depending on the level of the spellcaster. If Malar has MR 40%, for example, then he has a 40% chance of shrugging off a spell from anyone, be they a 1st-level spellcaster or a 21st-level spellcaster.