Strength is a combat stat in RuneScape. It is commonly used by warriors to hit higher with melee attacks. Strength does not fufill many of the weapon requirements except for halberds and is often trained to increase the max hits of warriors.
In PvP, Strength can be maxed to level 99 in order to hit very high with a lower level weapon, such as a level 40 rune scimitar or 2H sword. Players can get 40 Attack, 99 Strength, and 1 Defense and become "pures". Such pures can catch lower-level players offguard and easily get pk kills.