Res also called Resist, not to be confused with the similarly named Resistance, is the specific capacity to reduce damage from a particular non-standard damage source. The possible types of Res are Fire Res, Radiation Res, and Plasma Res for templars. Foes may have Bio-poison Res but not Radiation Res, as templars do not deal Radiation damage, but can deal Bio-poison damage.
Res is primarily granted by suits of armor, although some other gear can grant Res occasionally. Certain foes will always have resist to one or more elements.
When damage of a matching type to the Res is being dealt to the target, half of Res is subtracted from it immediately, and the other half is rolled between 1 and the half. So 12 Fire Res is rolled as 6 + (1d6) = 7..12
Fire Res in particular is special in that it reduces the heat cost of walking through burning tiles the same way it reduces the damage. Radiation Res is particularly valuable because many Xeno deal radiation damage that is otherwise hard to mitigate.
Foes with particular Res to an element will require special strategy to defeat. If your Hydra is facing a fire resist-heavy foe, you will very likely need assistance from another templar to take it down (possibly including a debuffing talent the Engineer has.)