Physical strength. How much damage you can do with melee weapons and how much you can carry. Also increases shove distance.
Strength increases the damage dealt by melee weapons and the distance a zombie is pushed back by a shove. The player's Max Carry Weight is also affected, at a rate of 20 lbs per point, unless the player has the Military Backpack, which increases it by 40 lbs per point. Each point of Strength also increases both the current and maximum level of the Blunt and Blades skills by 1 point, up to a maximum level of 127.