What the character's innate or natural resistances are vs wearing a ring/trinket or armor that provides a resistance...sort of like an enhancement to the wearer for the time that they have the item equipped. For example, the Chain Pro-fire has a 25% resistance to fire damage...there are some other armors that are pro fire, pro magic, pro gas and some other "pro's".
If you like, you can read through the equipment database which gives information on all the goods in the game.
Equipment Database
(Hey, so you are from Manchester NH? I don't live all that far from the border near Mason...I am in Mass. Probably about an hour or so from Manchester.)