Strength is one of your character stats. Strength points (SP) with Intelligence points also, make you a more productive worker. SP are used also in your Fight coefficient calculation.
You can grow your SP level by eating Meat (1piece of meat = +2SP, +2HP)
You must have 20 strength to obtain level 2 and 60 strength to obtain level 3. Strength also affects field productivity in wheat and corn fields.