All units belong to one of eight elements: Fire, Ice, Wind, Earth, Water, Lightning, Light, and Dark. Each element has its own advantages and disadvantages. Below, you can see the relationships between the elements, including their respective strengths and weaknesses.
When attacking with an elemental ability, units that are weak to that element will display a "Weak" icon above them, while units that are strong against that element will display a "Resist" icon. Additionally, each unit has an elemental resistance. Resistance affects the amount of damage they take when attacked. High resistance means they will take little damage when attacked with that element, while low resistance means the damage they take will be greater.
Tap the Map button on the Damage Prediction screen when attacking any time you want to see information on the different elements.