The amount of damage you take from an element = 100% - your resistance. If your resistance is negative, for example by wearing the antipode belt, then you take above-normal damage from the element. It is also possible for your resistance to go above 100, in which case you are healed by the element.
That was fully explained by the previous comment but I am happy to provide more detail. You want any resistance to be as high as possible and preferably over 100 percent if possible. Fire +50 reduces incoming fire damage by 50 percent. Cold -25 increases incoming cold damage by 25 percent. Fire +110 eliminates incoming fire damage and gives 10% of it back to you as healing. Cold -110 causes you take double damage plus an additional 10%.
A detail to add: the damage taken in a resistance calculation is rounded up for each individual damage packet. If you have 5% resistance (say, a low-level priest using Armor of Faith) and an enemy deals 15 damage of that type, you take 15*0.95=14.25, rounded up to 15 damage. That 5% resistance didn't help at all, and wouldn't for anything less than 20 damage. If you want to protect yourself from a source that deals damage in frequent packets of 1 each (like most poison or the acid pools in Durlag's Tower), you'll need full 100% resistance to help at all.
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