Cards and equipment can belong to one of four elements: Earth, Water, Fire, and Wind.  Each element is strong and weak to another.

Earth is effective against Water. Water is effective against Fire. Fire is effective against Wind. Wind is effective against Earth. 

Your knight's damage will change to reflect the element type of the equipment you are wearing. (Example: Ice mail has the water(30) attribute. Damage from your knight will be increased by 30% against fire enemies, doing a total of 130% damage. Valkyries and cards are NOT affected by knight's equipment.)  

In the case of having two different elements, the higher of the two will become your element. (Example: If you have both wind(50) and fire(30) items equipped, you will only have wind(50) for your elemental attribute.)  

Valkyries and cards each have their own elemental attribute. Most cards have an element of 50, giving them 50% bonus damage against a weaker element. (Example: Elle of Lonely Fire has fire(50) and will do 50% more damage against wind enemies, doing a total of 150% damage. Your knight is NOT affected by Valkyries and cards.) 

Please NOTE*: Elemental weakness will only increase damage. You will not have reduced damage from hitting a stronger element (Example: If you hit a water enemy with fire, your damage will not be reduced and you will still deal 100% damage.)