Part of the saving throw.
How It Works
MR1 would mean if you rolled the exact number needed as part of the save, you completely resist the magic. So if your MAG save is a 16, with MR1, you completely resist all effects on a 16.
If you have MR3, this means you can roll the target number or 2 higher to completely resist all effects (so a 16, 17 or 18 total).