Armour in Dark Souls provides different resistances to a number of different damage types, as of right now, there are six  damage types. Physical (split between slashing, piercing and bludgeoning) Fire, Lightning and Magic are the primary damage types and each has a corresponding AC modifier given by armour. Poison and Bleeding are status effects and certain sets of armour may allow advantage on saves against attacks that may apply these effects. If you are not wearing any armour you have disadvantage on saves against poison and bleeding. You have been warned.