What is it? Through the whole game I thought it meant that the higher the number the more a character would resist have a spell done to them and stuff like esuna or maybe cure would miss often. But when I go to optimize it says I should put on the black mask which raises my magic resist a lot. So is it good to have magic resist?

It means you take less damage from enemies' magic attacks. And I think you also evade status inducing spells more often, though I'm not certain on that.

Thanks, I always thought it was a bad thing to have a high magic resist but I guess it wouldn't make sense having a stat that makes you worse the higher it is.

I think it can be safely said that "magic resist" is enemy magic resist. Not your own beneficial spells.

Except for Reverse. You'd want a low magic resist (read: take of your helm) if you intend to cast reverse on yourself a lot. Although, there's no real need to do that in the normal story gameplay.

Even if you are using the reverse strategy the difference with and without a helm is marginal at most.

But when you are using reverse, it's probably for a really nasty beastie like Omega or Yiazmat, where you'd really want to have every (marginal) advantage working for you.

Nice, using the word "marginal" btw. (I'm a student of economics...it's a staple in my own vocab). *feeds sam a scooby snack*