In Magic School Lussid there are six different elements that Servants might have: 1) Fire 2) Thunder 3) Water (ice) 4) Dark 5) Light and 6) no-element.
Servants can do additional damage to the elements that are weak against them. For example: Fire element is strong against Thunder and deals +30% extra damage. Thunder does +30% damage to Water and Water does +30% damage to Fire. Dark and Light do +50% damage to each other and +10% to other elements (Fire, Thunder and Water). All elements do +10% damage to no-element.
While an element is strong against another element, they do less damage to the ones that are their weakness. In these situations the damage is reduced by -30%: Fire is Weak against Water. Water is weak against Thunder and Thunder is weak against Fire.
The Elemental Damage Cheat Sheet gives you a good view of the Elemental damage distribution in Magic School Lussid.