From Medivia Online Wiki

Strength is a stat only gotten through items, it increases your Melee Skill damage and Spellpower.
It can be used by knights and players with no vocation only.
The amount of strength you get from a certain item is directly related to its Item Rank.

One point of strength increases:
-minimum melee weapon damage by 0.30
-maximum melee weapon damage by 0.35
-spellpower by 1.20

Many equipment pieces and melee weapons provide Strength, often both as a Property and as a possible Attribute. Other items, such as the Green Witches Potion, Spicy Maris Feast and Melee Ring temporarily increase your dexterity.