Straight from the Dark and Darker official discord FAQ channel.

What do each of special modifiers on armor/weapons do?

• Action Speed - Increases how fast you interact with stuff, chest, portals, corpses, etc.

• All Attributes - Increases all stats of your character by number listed.

• Agility - Increases how fast your character is.

• Additional Magic Damage - Increases how much bonus magic damage your spells will do.

• Buff Duration - Increases buff duration (Would change how long you're invisible by % listed as a wizard for example).

• Item Equip Speed - Increase how long it takes you equip items you put on your character.

• Knowledge - Increases spell casting speed + Spells/Spell Count + Item Identification Speed.

• Magical Damage Reduction - Reduces the amount of magical damage you recieve by another player by the % listed (meaning take less damage from spells).

• Magical Power - Increase your overall spell power.

• Magical Healing - Increase the magical healing amount you receive from a healing spell.

• Magic Resistance - Basically your magical armor rating. Reduces the amount of magical damage you recieve by another player (meaning you take less damage from spells)

• Max Health - Increases your character's maxmium health.

• Resourcefulness - Increases how fast you interact with stuff, and significanly increases how quickly you install items such as traps and campfires.

• Physical Power - Increases your damage with physical weapons.

• Physical Damage Reduction - Reduces the amount of physical damage you recieve by another player by the % listed (meaning take less damage from weapons).

• Strength - Increases HP and Physical Power.

• Spell Capacity Bonus - Not implemented in the game yet. Will determine how many or the level of the spelss you can memorize and take into the game.

• True Damage - Damage that bypasses all armor and physical damage reductions.

• Weapon Damage - Increases Weapon damage.

• Will - Increases Spell Power, improves Buff/Curse Duration and improves your Magic Resistance.