Resistance To All
From Diablo Wiki
Resistance to All is a Unique Enchantment found in various items and charms in Diablo 2 that boosts the resistance of all 4 elements: Fire, Cold, Lightning, and Poison. The correct term used in the item database is "(X)% to All Resistances".
Resistance to All boosts the resistance of all the four elements that you frequently encounter in the Diablo 2 Realm, from traps to towers and monsters themselves. The amount of this resistance is the same for all elements allowing for less clutter on the weapon system and more organization. Resistance to All is most commonly found on all the Diamonds if socketed into a Shield, depended upon the perfection of the gem. The percent database does not add a certain percent of how much initial resistances you have, it is more of adding a percent resistance, not multiplying. If i was to have 10% percent resistances on all of my elements, and to come across a said item with "25% to all resistances", all the resistances will be 35 by addition, not 12.5 by multiplication.
The Resistances will dimish as you start to play on Nightmare and especially on Hell, so keep in mind that your resistances will change as you play on harder difficulties and attempt to search for higher resistance-based items. Also, to allow at least a steady amount of elemental damage be weighted upon you, your maximum resistance to any of the 4 elements is a steady 75. Although, other enchantments allow you to increase the maximum resistance of certain elements to provide even more elemental defense. There has not been an enchantment that allows the addition of a maximum resistance to all elements at once as of now.
All Resistances (or resistance to all) is very good for keeping a balanced check on your resistances without one standing out over the other, allowing for a better standoff at any situation involving elemental damage.