How does magic/spell resistance work? I mean, if I put on a certain belt that says Spell resistance: 12 does that mean that I am 12 percent resistant to magic? Or does it take away 12 % damage from spells? Or is it not a percent at all? Can someone explain this?

Let's say you have Spell Resistance: 12 from a belt.

When someone targets with you a spell, they must make a SR check ( 1d20 plus their casting level). The result of that roll must be equal or greater to your SR of 12 for the spell to hit you. If they fail, the spell is resisted and you suffer no ill effects from it.

Can this stack? Like if my helmet is magic resist: 5 and my belt is magic resist: 12 I have magic resistance: 17?

I'm not sure. Anyone?

Nope, the highest one applies. So you would get SR 12.