You are stronger than your size hints at. You are not larger than others, but you have a visibly athletic build. The trait is used whenever raw strength is used or of use, such as lifting things, tug-of-war, climbing, grappling etc. It increases your encumbrance limit by 1 kg per step of this trait. If you have +2, your encumbrance limit is 14 kg instead of 12 kg. It cannot be used in armed combat as that's more a case of technique than raw strength.
Cannot be taken if you have Weak.