On normal armor, resistances to status effects such as fire reduce the chances of said status from being inflicted, and also (if I'm not mistaken) reduce the damage of the effect.
So... What do status resistances do on shields? You can't get inflicted with a status if you're shielding, and if you get inflicted otherwise, raising your shield does nothing to circumvent the status effect.
Well, shock is completely negated in terms of damage by your shield, so far I've seen.
Fire I think is reduced.
I'm not sure, but I think the shield is resistant to the damage inflicted by the status, as well as any attack that hits you bearing that status, like a Fireball will be resisted better by a shield with Fire defense than a shield with no defense.
Also a shock resist shield should take less damage from the shock effect than a normal shield would.
I THINK.