Full bootypedia article on the subject:
'A class of items which boost Resistances of worn armor.
These items usually fit only certain slots in warrior's equipment, and start to work only the next half-turn after equipping. Effects usually stack unless items share same Gear Type, and even then, only Resistance Factor (RF) doesn't stack. RF is spelled as follows:
>Effectivity: by what factor Resistance is improved between current value and Boundary;
>Boundary: if the current resistance is at this level or better, effect is zero. By stacking, you can approach this value, with diminishing returns, but never reach it.
Lower numbers are better.'
There's a lot going on in one article. Just remember what legion has highlighted; the second number is the floor, and should be compared to the resistance value of your armour. If your armour already has a resistance lower than the floor, then the item is useless to you.
And no, you can't wear multiple hats. And even if you do find a way to equip multiple hats, you'll only get the benefit from one of them.