At the start of Terrain of Magical Expertise, you will create your character. You have the option to choose your class as well as your element. Each element has another that it is strong against and one it is weak against.
The five elements of Terrain of Magical Expertise are Natural, Technical, Crystal, Infernal, and Neutral.
Attacks that are strong against another element will deal additional damage. Attacks that resisted will deal less damage to a creature of an element that the attack is weak against. Neutral attacks have no weaknesses and no strengths so are effective against all elements. Using a combination of elements on your team is crucial.
Sometimes an elemental attack can cause a status effect to an enemy. Infernal can curse the enemy which makes them take damage over time. Natural can turn an enemy into a flower for a few turns where they can only attack and defend. Crystal encases the enemy in ice where they are helpless until they are hit. Technical can paralyze an enemy which reduces their stats and may forfeit their turn. Some enemies will have immunity to certain status effects.
For more information on Terrain of Magical Expertise, check out How to scan enemies in Terrain of Magical Expertise and All Companions in Terrain of Magical Expertise on Pro Game Guides.